Plugin disabling logic was broken by e009d3e4. Prior to this change,
a plugin would be disabled if it or any of the implemented interfaces
in its inheritance hierarchy were configured as being disabled. The
offending commit inverted the logic so that the plugin would be
enabled if any part of it was NOT configured as being disabled.

This commit restores the logic such that the early return happens only
in the negative case.

Previously, the tests were written as though
PluginContext#getPlugin(Class) would consider the specified class
against the runtime type of the plugin (not an unreasonable
assumption); rather this method considers the broader 'plugin type'.
This commit rewrites the test to seek by plugin type and assert the
absence of the disabled plugins.

Closes gh-5032