Skip time-related test if we are running via MySQL

Closes #1341

The thing about this commit is that MySQL itself supports microsecond precision
just fine. But the Django ORM does not support that through MySQL.

Luckily, we still run this test when we operate with sqlite, so it is reasonably
reasonable to skip it in the case that we are in MySQL mode.
Merge pull request #1372 from paulproteus/bugfix-1341

Skip time-related test if we are running via MySQL