Add Markdown feature specs for markup inside links

It was already working, just want to make sure we don't break it in the
future.

spec/features/markdown_spec.rb

@@ -161,6 +161,19 @@ describe 'GitLab Markdown' do
       end
     end
 
+    describe 'Edge Cases' do
+      it 'allows markup inside link elements' do
+        expect(@doc.at_css('a[href="#link-emphasis"]').to_html).
+          to eq %{<a href="#link-emphasis"><em>text</em></a>}
+
+        expect(@doc.at_css('a[href="#link-strong"]').to_html).
+          to eq %{<a href="#link-strong"><strong>text</strong></a>}
+
+        expect(@doc.at_css('a[href="#link-code"]').to_html).
+          to eq %{<a href="#link-code"><code>text</code></a>}
+      end
+    end
+
     describe 'EmojiFilter' do
       it 'parses Emoji' do
         expect(@doc).to have_selector('img.emoji', count: 10)

spec/fixtures/markdown.md.erb

@@ -94,6 +94,14 @@ The problem with SanitizationFilter is that it can be too aggressive.
 | `1 < 3 & 5` | 1 &lt; 3 &amp; 5 | 1 < 3 & 5 |
 | `<foo>`     | &lt;foo&gt;      | <foo>     |
 
+### Edge Cases
+
+Markdown should be usable inside a link. Let's try!
+
+- [_text_](#link-emphasis)
+- [**text**](#link-strong)
+- [`text`](#link-code)
+
 ### EmojiFilter
 
 Because life would be :zzz: without Emoji, right? :rocket: