Support testing plugins

It's currently difficult to test plugins, because they're not always built.
We can only test them via the API by which they're used, testing via whatever Qt component they support (be that SSL, SQL, platform QPA or whatever). This can be somewhat indirect, making it difficult to exercise the particular piece of code one has fixed, for the sake of a regression test, for example.

It should be possible to, for example, put tests in qtbase/tests/auto/plugins/platforms/android/ – that are built and testable precisely when qtbase/src/plugins/platforms/android/ is built – that test the internals of the plugin, e.g. to catch regressions, or to systematically unit test some internal API that the public API of the plugin only accesses indirectly (which commonly makes it hard to control exactly how a particular part of its internals gets exercised).
The test binary for that would need to dynamically link to the plugin, of course.