tests: add global test_context_fixture

Move the existing app_test_context_fixture from test_pullrequests.py to
conftest.py to make it available to all test modules.

It is useful in two cases:

1. there is test setup code (xUnit style) that needs to execute in the same
test context as the actual test.

2. even without test setup code, an entire test needs to be executed in a
test context. In this case, the fixture just reduces code complexity by not
requiring changes in the test code (compared to standard 'with
test_context').

It is possible to apply this (or any) fixture to an entire test class using
the class decorator
@pytest.mark.usefixtures("...")
This is similar to 'autouse=True' but can be used even if the fixture is
defined elsewhere.