Use double comment markers and full stop

Co-authored-by: James Henderson <[email protected]>

Author: rpatel321

llvm/utils/lit/tests/xunit-output-report-failures-only.py

@@ -1,6 +1,6 @@
 # UNSUPPORTED: system-windows
 
-# Check xunit output
+## Check xunit output.
 # RUN: rm -rf %t.xunit.xml
 # RUN: not %{lit} --report-failures-only --xunit-xml-output %t.xunit.xml %{inputs}/xunit-output
 # If xmllint is installed verify that the generated xml is well-formed