[lit] Pass the COMSPEC variable through to test processes on Windows

This variable is necessary for the system() function for running
external processes.

This is needes for some libcxx tests. With the current libcxx test
infrastructure, all OS environment variables are passed through, but
with the new "from scratch" libcxx test setup, we only pass through
the variables listed here.

Differential Revision: https://reviews.llvm.org/D115363