If QUBES_TEST_TEMPLATES or QUBES_TEST_LOAD_ALL is set, create testcases
on modules import, instead of waiting until `load_tests` is called.
The `QUBES_TEST_TEMPLATES` doesn't require `qubes.xml` access, so it
should be safe to do regardless of the environment. The
`QUBES_TEST_LOAD_ALL` force loading tests (and reading `qubes.xml`)
regardless.
This is useful for test runners not supporting load_tests protocol. Or
with limited support - for example both default `unittest` runner and
`nose2` can either use load_tests protocol _or_ select individual tests.
Setting any of those variable allow to run a single test with those
runners.
With this feature used together load_tests protocol, tests could be
registered twice. Avoid this by not listing already defined test classes
in create_testcases_for_templates (according to load_tests protocol,
those should already be registered).
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