Ensure all tests that create TaskInstances in DB also create DagRuns

[ashb]

As part of the work on AIP-39 we want to make it so that every TaskInstance in the DB has DagRun.

Now at "run time" this is already the case (baring the case where someone might delete a DagRun. Rare, now only possible by direct DB? I can't remember if Browse -> DagRuns allows deleting anymore or not) but in our tests this is far from the case

There are countless examples of where the tests just directly create TaskInstances, without a matching DagRun.

We need to fix that so that the tests reflect reality.

• Add Pytest fixture to create dag and dagrun and use it on local task job tests #16889 is part of this -- creating a pytest fixture to reduce amount of duplication in tests
• Use dag_maker fixture in some test files under tests/models #17556
• We probably also want to enforce foreign key constraints between DagRun and TaskInstance once we've done this (see also Check for missing DagRun rows for "downstream" tables before migrating DB #17030)