Set HOME for tests that use module cache path

Getting the default module cache path calls llvm::sys::path::cache_directory,
which calls home_directory, which checks the HOME environment variable
before falling back to getpwuid.  When compiling against musl libc,
which does not support NSS, and running on a machine that doesn't have
the current user in /etc/passwd due to NSS, no home directory can
be found.  Set the HOME environment variable in the tests to avoid
depending on getpwuid.

Reviewed By: pirama, srhines

Differential Revision: https://reviews.llvm.org/D132984