[llvm-cov] Try to fix the native_separators.c test some more

It's still breaking this bot (though, it looks like it always had been):

  http://lab.llvm.org:8011/builders/clang-x86-windows-msvc2015

This time, add quotes around llvm-{cov,config} so that lit won't expand
them.

Thanks to Reid for suggesting the patch!

llvm-svn: 281072