[libcxx] [test] Pass some windows environment variables through to test processes

Normally, the run.py wrapper script runs the child processes in
a clean environment, with only the environment variables available
that are passed via the --env parameter.

However, the COMSPEC and TEMP variables are kind of necessary when
running some tests; COMSPEC is necessary for finding the interpreter
when executing commands via std::system().

Before f1a96de1, tests were executed
via an intermediate shell which implicitly readded the COMSPEC variable.

The TEMP variable allows temp files to be placed in a sensible
location; if unset, they're placed in the default temp fallback of
C:\Windows instead.

Differential Revision: https://reviews.llvm.org/D97452