mimeview tests: make it possible to run the trac.mimeview.tests.api tests in isolation.
mimeview tests: make it possible to run the trac.mimeview.tests.api tests in isolation.

When run as part of the whole test suite, the `Converter[123]` components had the name `trac.mimeview.tests.api.converter[123]`, so they got enabled thanks to having the `trac.` prefix. When run directly, they had the name `__main__.converter[123]`, so they were not enabled, and `converters` extension point returned an empty list.

pkdevbox/trac,pkdevbox/trac,pkdevbox/trac,pkdevbox/trac