FileBackendIntegrationTest was running tests against different backends
in an unconventional way, using a combination of wrapper test cases that
run tests against two different classes, and CLI options which don't
really exist anymore and have an associated fixme.
So:
* Move the bulk of FileBackendIntegrationTest to a new abstract base
class under tests/phpunit/integration.
* Add subclasses for the FS and multiwrite test cases. This allows us to
eliminate the wrappers.
* Add a subclass for MemoryFileBackend.
* Add a Swift subclass which replaces the main use case for
the CLI option --use-filebackend. It is automatically enabled when
a Swift backend is configured, similar to PostgreSQL tests.
* Some miscellaneous tests with a medium level of integration, not
requiring backend setup and teardown, were moved to new classes
FileBackendMultiWriteTest and FileBackendStoreTest.
Change-Id: I0da531349d7627970a7bcb34f3c1f5fd7e05cb21
