Add a base class for ITs in plugins that arranges for it to be loaded.

It really wasn't feasible for the plugin code itself to arrange for this to happen, so the only way to test was to install it into the server directory. Which meant that we couldn't have any serious plugins (well, perhaps the MySQL adapter).

Refactor plugins a tiny bit to allow for one that uses the default class loader.

Make the test harness obey the configured plugins finder rather than overriding it unconditionally. Default to none, as before.

See https://github.com/MMcM/sql-layer-plugin-example for an example using this.