[libfuzzer] Enable trace-malloc-unballanced.test on Windows.

Summary:
Enables trace-malloc-unbalanced.test on Windows, fixing two problems it had with Windows before.
The first fix is specifying python instead of relying on a script's shebang since they can't be used on Windows.
The second fix is making the regex tolerate windows' implementation of the "%p" format string.

Reviewers: Dor1s

Reviewed By: Dor1s

Subscribers: morehouse

Differential Revision: https://reviews.llvm.org/D51760

llvm-svn: 341632