[libFuzzer] Fix test because cmd prompt does not expand wildcard.

[libFuzzer] Fix test because cmd prompt does not expand wildcard.

Commands should expand the wildcards on Windows, the cmd prompt doesn't.
Because of that sancov was not finding the needed file.
To deal with this, we use ls and xargs from gnu win utils.

Differential Revision: https://reviews.llvm.org/D29374

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@293825 91177308-0d34-0410-b5e6-96231b3b80d8