Added test for #5053.

Note that the fix for #5099 also fixes #5053, a pandoc 2.4
regression in parsing underscore emphasis after symbols.

test/command/5053.md

@@ -0,0 +1,14 @@
+```
+% pandoc
+—_legibility_—
+^D
+<p>—<em>legibility</em>—</p>
+```
+
+```
+% pandoc
+_filename_|_filetype_
+^D
+<p><em>filename</em>|<em>filetype</em></p>
+```
+