Don't run background-index.test on Windows.

The test had a "UNSUPPORTED: win32" line, but the spelling of that
changed in r339307 a year ago. Finally update this test too.

Part of PR43592.

llvm-svn: 374745

clang-tools-extra/clangd/test/background-index.test

@@ -1,5 +1,5 @@
 # We need to splice paths into file:// URIs for this test.
-# UNSUPPORTED: win32
+# UNSUPPORTED: windows-msvc
 
 # Use a copy of inputs, as we'll mutate it (as will the background index).
 # RUN: rm -rf %t