assert-test-is-not-parallel does not work inside fixtures

[camsaul]

From metabase/metabase#26843

My original description:

We have logic to make sure you don't do things like with-redefs inside ^:parallel tests. We do this by swapping out clojure.core/test-var with a version that binds a *parallel? dynamic var, and that's what assert-test-is-not-parallel checks. However, this doesn't affect :each fixtures, since the each fixture logic happens before test-var ever gets called...

I think we need to intercept clojure.test/test-vars as well and maybe wrap the :each fixtures to bind *parallel?* to get it to work properly... I tried this but it's a little tricky because the :each fixtures don't get passed the vars themselves, but get passed the test function (:test metadata on the var). Figuring out how to do this will be a little tricky, but ultimately doable I think

@bshepherdson also suggested

Perhaps we can check this in both directions - keep the test-var logic we have, plus cause parallel-unsafe things like with-redefs to set another dynamic var that gets checked on the way in to a test-var. Then if a fixture has already done something unsafe, but we're starting a parallel test, we can assert.