tests: reduce the number of application fixtures

[jacquerie]

Every application fixture is a spot in which we have to specify the application configuration, which has already led to (minor) divergences between configurations. Let's kill this before it becomes a problem by having exactly one application fixture per test suite.

If a test requires a certain configuration variable to be set, that should happen in the preamble of the test, not in the fixture.

• [x] email_app and app should be merged in test/unit/conftest.py
• [x] small_app and app should be merged in tests/integration/conftest.py
• [ ] workflow_app from tests/integration/test_arxiv_workflow.py and app from tests/integration/conftest.py should be merged

[jacquerie]

Touching the difficulty up to hard, because merging small_app and app requires to change the scope of the fixture from function to session, which implies some careful work to avoid persisting modifications from one test to the other.

[jacquerie]

Commit https://github.com/inspirehep/inspire-next/commit/c97fb948a423a3ac9f09b50010863b19d85d1a13 introduced another application fixture (workflow_app).