gtest runner: now always does a clean step after running.

* Checks for any 'clean' arg, setting clean-only mode.
* If clean-only mode, skip the make.
* Adds the equivalent of os.devnull in a way that supports the file-like 'write' operation.
* Uses a null sysout/syserr for the clean step so we don't see noise from cleaning.
* Now always runs the clean step after a real test run so we don't have cruft left over after a test run.

llvm-svn: 218871