Benchmark.t: test for inconsistent clock

test 15 has been failing intermittently in smokes for ages now.
This does countit(3, ...) and countit(1, ...) and checks that the first
takes approx three times longer than the second.

This commit adds in near the beginning a crude timing loop that directly
uses times() rather than anything in the Benchmark module, and that does
approx 1s and 3s of loops, and if the results aren't consistent, sets a
global flag, $INCONSISTENT_CLOCK, that causes timing-sensitive tests to be
skipped.

For now it only skips test 15. If this is successful, I'll look to
expand it to other failing tests like 128/129.