This creates a dedicated test suite for integration tests. All of them
can be run independently, and there is no order dependency - previously,
all integration tests needed the installer test to run first, and they
would fail if installation failed.
Now, the developer will have to set up a Flarum database to be used by
these tests. A setup script to make this simple will be added in the
next commit.
Small tradeoff: the installer is NOT tested in our test suite anymore,
only implicitly through the setup script. If we decide that this is a
problem, we can still set up separate, dedicated installer tests which
should probably test the web installer.
