Testsuite cleanup can delete everything

If you run ./hadrian/build test and press Ctrl-C at an unfortunate moment, it deletes the entire repository (!).

I encountered this by accident. After looking at the traceback and adding pauses in random places, I reproduced it a couple more times, but it is rare.

When the test ends, we call cleanup() in testlib.py:

shutil.rmtree(str(testdir), ignore_errors=False)

where the test directory in testglobals.py is initialized to:

self.testdir = Path('.')

The code later overwrites testdir with a temporary directory. Unless you press Ctrl-C.