tests: add test for non-integer delay timers.

PR #8034 came with a test to make sure that timers expiry is based on
monotonic time and not on wall-clock time. However, a bug in the
implementation broke timers with non-integer delays. A fix for this
issue was provided with PR #8073, but it didn't come with a test.

Because #8073 fixed a subtle issue that could reappear in the future,
and because the impact of such an issue would be significant, I suggest
adding this test.

The test would timeout after 1 minute if the issue was reproduced.
Otherwise it will run very quickly.

Reviewed-By: Fedor Indutny <fedor@indutny.com>