Merge #37072

37072: tests: avoid t.Parallel() when the top test has defers r=tbg,RaduBerinde a=knz

Sub-tests that invoke `t.Parallel()` get to run concurrently with
their parent test, and may be delayed arbitrarily past beyond the end
of the termination of the parent test (including beyond the execution
of its `defer` calls).

This means it's unsafe to call `t.Parallel()` with
e.g. `leaktest.AfterTest()`.

Release note: None

Co-authored-by: Raphael 'kena' Poss <knz@cockroachlabs.com>

Author: craig[bot]