[test] Create a test topsite db for testing webhooks

[karlcow]

Some tests have a dependency on getting the right labels. If a test makes a query to a populated sites.db, the test will return a different set of labels.

There are two options:

1. mocking everywhere we request the labels.
2. Create a db when we are running tests with the right information in it.

Option 2 seems a better choice. We could rationalize it to a couple of controlled domains for our tests.

• [ ] Create a fake db in unittests levels
• [ ] Make sure the tests are using the right domains for this to be effective
• [ ] Remove the mocking of extract_priority_label if it still exists

This mainly impacts webhooks tests

[karlcow]

See https://github.com/webcompat/webcompat.com/pull/3167/commits/f5ad195fc293fdf0504b0fd635ecf32f14aa7927 for the origin of this requirement.