Force SHELL to be cmd.exe on Windows for the test suite

Summary:
Windows make will search for other shells and choose those over cmd
if available (e.g. C:\cygdrive\bin\sh.exe). This shell has numerous
issues with path handling (/ vs \\ vs \ and C:). So default to using
cmd.exe which is known to work.

Reviewers: zturner, sas, xiaobai

Reviewed By: zturner

Differential Revision: https://reviews.llvm.org/D54510

llvm-svn: 346993