Adds MyAdmin\Plugins\Testing: PluginContractTestCase (17 contract inspectors),
ServicePluginTestCase (activate/deactivate/queue behavioral suite), the fake
container (FakeDb, FakeSettings, ConstantStub, recording fakes), Bootstrap::init()
and tools/fleet-matrix.php.

Additive only — no behavioral change to Plugin, Installer, PluginScanner,
VendorGuard, Loader, modules.php or function_requirements.php. First tagged
release since v2.0.2 (2021-03-19); also releases the 10 master commits that the
fleet has been consuming via dev-master.

Consumers move from dev-master to ^2.1.