[GITEA] Add footnote testing

- This adds coverage to the most common and the edge cases of what the
footnote implementation should be capable of. This was partly done to
ensure no hidden surprises when changing the implementation, as markdown
rendering is one of the more important features of Forgejo.

(cherry picked from commit 16ecdb4170)
(cherry picked from commit 19dc5ef5e5)
(cherry picked from commit d5955efc0a)
(cherry picked from commit 2cdaf10836)
(cherry picked from commit 251b567794)

Conflicts:
	modules/markup/markdown/markdown_test.go
	https://codeberg.org/forgejo/forgejo/pulls/2153
(cherry picked from commit f863f4b005)

modules/markup/markdown/markdown_test.go

@@ -546,6 +546,204 @@ func TestMathBlock(t *testing.T) {
 	}
 }
 
+func TestFootnote(t *testing.T) {
+	testcases := []struct {
+		testcase string
+		expected string
+	}{
+		{
+			`Citation needed[^0].
+[^0]: Source`,
+			`<p>Citation needed<sup id="fnref:user-content-0"><a href="#fn:user-content-0" rel="nofollow">1</a></sup>.</p>
+<div>
+<hr/>
+<ol>
+<li id="fn:user-content-0">
+<p>Source <a href="#fnref:user-content-0" rel="nofollow">↩︎</a></p>
+</li>
+</ol>
+</div>
+`,
+		},
+		{
+			`Citation needed[^0]`,
+			`<p>Citation needed[^0]</p>
+`,
+		},
+		{
+			`Citation needed[^1], Citation needed twice[^3]
+[^3]: Source`,
+			`<p>Citation needed[^1], Citation needed twice<sup id="fnref:user-content-3"><a href="#fn:user-content-3" rel="nofollow">1</a></sup></p>
+<div>
+<hr/>
+<ol>
+<li id="fn:user-content-3">
+<p>Source <a href="#fnref:user-content-3" rel="nofollow">↩︎</a></p>
+</li>
+</ol>
+</div>
+`,
+		},
+		{
+			`Citation needed[^0]
+[^1]: Source`,
+			`<p>Citation needed[^0]</p>
+`,
+		},
+		{
+			`Citation needed[^0]
+[^0]: Source 1
+[^0]: Source 2`,
+			`<p>Citation needed<sup id="fnref:user-content-0"><a href="#fn:user-content-0" rel="nofollow">1</a></sup></p>
+<div>
+<hr/>
+<ol>
+<li id="fn:user-content-0">
+<p>Source 1 <a href="#fnref:user-content-0" rel="nofollow">↩︎</a></p>
+</li>
+</ol>
+</div>
+`,
+		},
+		{
+			`Citation needed![^0]
+[^0]: Source`,
+			`<p>Citation needed<sup id="fnref:user-content-0"><a href="#fn:user-content-0" rel="nofollow">1</a></sup></p>
+<div>
+<hr/>
+<ol>
+<li id="fn:user-content-0">
+<p>Source <a href="#fnref:user-content-0" rel="nofollow">↩︎</a></p>
+</li>
+</ol>
+</div>
+`,
+		},
+		{
+			`Trigger [^`,
+			`<p>Trigger [^</p>
+`,
+		},
+		{
+			`Trigger 2 [^0`,
+			`<p>Trigger 2 [^0</p>
+`,
+		},
+		{
+			`Citation needed[^0]
+[^0]: Source with citation needed[^1]
+[^1]: Source`,
+			`<p>Citation needed<sup id="fnref:user-content-0"><a href="#fn:user-content-0" rel="nofollow">1</a></sup></p>
+<div>
+<hr/>
+<ol>
+<li id="fn:user-content-0">
+<p>Source with citation needed<sup id="fnref:user-content-1"><a href="#fn:user-content-1" rel="nofollow">2</a></sup> <a href="#fnref:user-content-0" rel="nofollow">↩︎</a></p>
+</li>
+<li id="fn:user-content-1">
+<p>Source <a href="#fnref:user-content-1" rel="nofollow">↩︎</a></p>
+</li>
+</ol>
+</div>
+`,
+		},
+		{
+			`Citation needed[^#]
+[^#]: Source`,
+			`<p>Citation needed<sup id="fnref:user-content-1"><a href="#fn:user-content-1" rel="nofollow">1</a></sup></p>
+<div>
+<hr/>
+<ol>
+<li id="fn:user-content-1">
+<p>Source <a href="#fnref:user-content-1" rel="nofollow">↩︎</a></p>
+</li>
+</ol>
+</div>
+`,
+		},
+		{
+			`Citation needed[^0]
+    [^0]: Source`,
+			`<p>Citation needed[^0]<br/>
+[^0]: Source</p>
+`,
+		},
+		{
+			`[^0]: Source
+
+Citation needed[^0].`,
+			`<p>Citation needed<sup id="fnref:user-content-0"><a href="#fn:user-content-0" rel="nofollow">1</a></sup>.</p>
+<div>
+<hr/>
+<ol>
+<li id="fn:user-content-0">
+<p>Source <a href="#fnref:user-content-0" rel="nofollow">↩︎</a></p>
+</li>
+</ol>
+</div>
+`,
+		},
+		{
+			`Citation needed[^]
+[^]: Source`,
+			`<p>Citation needed[^]<br/>
+[^]: Source</p>
+`,
+		},
+		{
+			`Citation needed[^0]
+[^0] Source`,
+			`<p>Citation needed[^0]<br/>
+[^0] Source</p>
+`,
+		},
+		{
+			`Citation needed[^0]
+[^0 Source`,
+			`<p>Citation needed[^0]<br/>
+[^0 Source</p>
+`,
+		},
+		{
+			`Citation needed[^0] [^0]: Source`,
+			`<p>Citation needed[^0] [^0]: Source</p>
+`,
+		},
+		{
+			`Citation needed[^Source here 0 # 9-3]
+[^Source here 0 # 9-3]: Source`,
+			`<p>Citation needed<sup id="fnref:user-content-source-here-0-9-3"><a href="#fn:user-content-source-here-0-9-3" rel="nofollow">1</a></sup></p>
+<div>
+<hr/>
+<ol>
+<li id="fn:user-content-source-here-0-9-3">
+<p>Source <a href="#fnref:user-content-source-here-0-9-3" rel="nofollow">↩︎</a></p>
+</li>
+</ol>
+</div>
+`,
+		},
+		{
+			`Citation needed[^0]
+[^0]:`,
+			`<p>Citation needed<sup id="fnref:user-content-0"><a href="#fn:user-content-0" rel="nofollow">1</a></sup></p>
+<div>
+<hr/>
+<ol>
+<li id="fn:user-content-0">
+ <a href="#fnref:user-content-0" rel="nofollow">↩︎</a></li>
+</ol>
+</div>
+`,
+		},
+	}
+	for _, test := range testcases {
+		res, err := markdown.RenderString(&markup.RenderContext{Ctx: git.DefaultContext}, test.testcase)
+		assert.NoError(t, err, "Unexpected error in testcase: %q", test.testcase)
+		assert.Equal(t, test.expected, res, "Unexpected result in testcase %q", test.testcase)
+	}
+}
+
 func TestTaskList(t *testing.T) {
 	testcases := []struct {
 		testcase string