quartz: make clock test less flaky

[Dan Kegel]

Fairly often, 'make test' failed on my machine with

referenceclock.c:70: Test failed: SystemClock - Expected difference
around 10000000, got 9960000
make[2]: *** [referenceclock.ok] Error 1

The attached patch relaxes the test slightly, and makes it pass reliably on my
thinkpad x60 on gutsy.
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