Add withTestNow() for scoped time mocking #583

Job	Run time
cs-stan / Coding Standard & Static Analysis	30s
testsuite / testsuite-linux (8.5, highest, false)	18s
testsuite / testsuite-linux (8.2, highest, false)	17s
testsuite / testsuite-linux (8.3, highest, false)	14s
testsuite / testsuite-linux (8.2, lowest, false)	14s
testsuite / testsuite-linux (8.4, highest, false)	13s
	1m 46s

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