Fix utils/datetime_test:timestamp__subtraction

Don't expect a negative time delta to throw an exception--instead, expect
1 microsecond to be returned per the behavior change made in r436312.

Approved by: brooks (as part of a larger diff), (maintainer timeout)
Differential Revision: D10316
PR: 217973
Reported by: Jenkins
Reviewed by: asomers
Sponsored by: Dell EMC Isilon