This commit changes to run test_plugins in a separate process.
test_plugins touches urlpatterns and Site registry during setUp(),
but the cleanup process in tearDown does not recover urlpatterns correctly.
After tests under test_plugins run, the urlpatterns continues to have
entries of test panels and this leads to NoReverseMatch error when
rendering other panels after that.
I investigated details in setUp/tearDown of PluginTestCase but failed
to find a good solution to recover urlpatterns and Horizon Site registry.
As a workaround, runnig test_plugins in a separate process can avoid
the issue.
Closes-Bug: #1809983
Change-Id: I848f6b341b3f93ed055955b2b12d2497811edc5a
a69ba853a7