[llvm] [Windows][test] Fix "LLVM" test failures when LLVM_WINDOWS_PREFER_FORWARD_SLASH is ON (PR #184556)

[via llvm-commits]

================
@@ -23,13 +23,13 @@
 
 
 
-# HTML-TOP: coverage\index.html
+# HTML-TOP: coverage{{[/\\]}}index.html
----------------
yhgu2000 wrote:

> Maybe directory_coverage.linux.test and directory_coverage.win.test could be merged? It seems the slash direction is the only difference?

Yes, it is the only difference. Sorry for the late reply.

https://github.com/llvm/llvm-project/pull/184556