[llvm-cov] Speculate fix for a Windows-only test (NFC)

This test should have broken after r280896. Fix up the test case
speculatively, since I don't have a way to test it.

I wonder why I didn't get any angry bot emails about this. Maybe none of
the win32 bots test llvm-cov? That could explain it, since the test says
it 'REQUIRES: system-windows', which is restricted to win32 hosts.

Also: why is 'system-windows' not defined for non-win32 Windows bots?
llvm-svn: 281008