tests/kernel/context: Fix sleep test for fast ticks

The sleep test was checking that the sleep took no longer than "2
ticks" longer than requested.  But "2 ticks" for fast tick rate
configurations can be "zero ms", and for aliasing reasons it's always
possible to delay for 1 unit more than requested (becuase you can
cross a millisecond/tick/whatever boundary in your own code on either
side of the sleep).  So that "slop" value needs to be no less than
1ms.

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