[test] Fix propagating HOME envvar to unittests

[mgorny]

Summary

Propagate HOME environment variable to unittests. This is necessary
to fix test failures resulting from pw_home pointing to a non-existing
directory while being overriden with HOME. Apparently Gentoo users
hit this sometimes when they override build directory for Portage.

Original bug report: https://bugs.gentoo.org/674088

[chandlerc]

I mean, this is gross, but sure, LGTM.

[mgorny]

@vitalybuka, regarding http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-fast test failures, could you verify whether the buildbot is not passed invalid HOME?

[vitalybuka]

Somehow sanitizers push buildmaster environment to slaves. Fixed with r350268. It requires buildmaster restart, so I am going to add a workaround to make it work before that happen.

[vitalybuka]

FYI @eugenis @pcc

[mgorny]

Thank you for addressing this!

test/Unit/lit.cfg.py

 
 # Propagate the temp directory. Windows requires this because it uses \Windows\
 # if none of these are present.
 if 'TMP' in os.environ:
     config.environment['TMP'] = os.environ['TMP']
 if 'TEMP' in os.environ:
     config.environment['TEMP'] = os.environ['TEMP']
 
+# Propagate HOME as it can be used to override incorrect homedir in passwd
+# that causes the tests to fail.
+if 'HOME' in os.environ:
+    config.environment['HOME'] = os.environ['HOME']
+
 # Propagate path to symbolizer for ASan/MSan.
 for symbolizer in ['ASAN_SYMBOLIZER_PATH', 'MSAN_SYMBOLIZER_PATH']:
     if symbolizer in os.environ:
         config.environment[symbolizer] = os.environ[symbolizer]
 
 # Win32 seeks DLLs along %PATH%.
 if sys.platform in ['win32', 'cygwin'] and os.path.isdir(config.shlibdir):
     config.environment['PATH'] = os.path.pathsep.join((
             config.shlibdir, config.environment['PATH']))
 
 # Win32 may use %SYSTEMDRIVE% during file system shell operations, so propogate.
 if sys.platform == 'win32' and 'SYSTEMDRIVE' in os.environ:
     config.environment['SYSTEMDRIVE'] = os.environ['SYSTEMDRIVE']