(core) Fix flaky SQLiteDB pause test: don't assert that unpaused writes are fast

Summary:
'should honor pauses' asserted that writes complete within 200ms, which is a
claim about the machine rather than about pause(). The check after unpause()
fails too often - unpause() releases the write held from before, so one 200ms
budget had to cover two fsync'd CREATE TABLEs. Writes that should happen are
now simply awaited, and the mocha timeout raised.

The problem is disk contention, not CPU: against 16 fsync'ing writers, with no
CPU load at all, the old test fails 24/25 and the new one 0/25.

Test Plan:
this is a pure test change, with flakage replicated against heavy
sources of disk contention.

Reviewers: Spoffy

Reviewed By: Spoffy

Differential Revision: https://phab.getgrist.com/D5015