Reorganise testing and CI

[JHopeCollins]

Bit of a reorganisation of the testing and CI.

Testing changes:

1. Split into unit tests and integration tests. Unit tests shouldn't depend on any other module, and the integration tests require multiple.
2. The tests using CirculantPC with the AllAtOnceSolver have been removed from tests/units/allatonce - this means that the tests in tests/unit/allatonce doesn't depend on complex_proxy. The removed tests have been added to tests/integration

CI changes:

1. Automatically open PRs to update github actions with dependabot.yml, e.g. action/checkout@v2 is two versions out of date.
2. Automatically run the CI tests over the weekend with a cron job to see if any upstream changes have broken things for us
3. In the CI workflow, run the unit tests, then the integration tests. If any of the unit tests fail then the integration tests will almost certainly fail, so this stops them running unnecessarily. The pytest-coverage is cumulative over both runs so the coverage report at the end should be accurate.

Note that the tests won't pass until the Firedrake docker has rebuilt after firedrakeproject/firedrake#3895 merged into Firedrake master. This is now finished.