tests: avoid t.Parallel() when the top test has defers

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()`.

This patch removes `t.Parallel` from the test in
`sql/physical_props_test.go` and modifies
`pkg/rpc/TestGRPCKeepaliveFailureFailsInflightRPCs` to use a wait
group instead.

Release note: None

Author: knz