Tests for TempFSFile

Tests for TempFSFile

These are in preparation for making a TempFSFileFactory service, thus
the odd break-up into two files. I split it into a separate commit so
that we could verify that the same tests pass before and after the
conversion to service.

Tests cover everything except getUsableTempDirectory() (which I don't
see how to test), and register_shutdown_function()-related stuff (which
seems actually impossible to test without starting a new PHP process).

Change-Id: If61b7ea3e332adc2bceefc8e6879a9e9443c99dd