tests/kernel/common: Fix uptime delta test for fast ticks

The test for the k_uptime_delta utilities was calling it in a loop and
waiting for the uptime to advance.  But the code was specifically
wanting it to advance 5ms or more at one go, which clearly isn't going
to work for a tick rate above 200 Hz.

The weird thing is that the test knew this and even commented about
the limitation.  Which seems silly: it's perfectly fine for the clock
to advance just a single millisecond.  That's correct behavior too.
Let's test for that, and it will work everywhere.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>