Create reference() tests for intervals of 1 ms.

Test 1 (lower bound): 2 periods of 1ms
Test 2 (lower bound): 3 periods of 1ms
Test 3 (high bound): 180000 periods of 1ms
Test 4 (middle): 4321 periods of 1ms

In TimeScales, rounding down intervals of <=1 ms will now give a time period of 1 ms, rather than null because intervals of <= 1 ms are valid time periods, but we just cannot go more precise than that.