SR-2856: adapt dispatch_timer_set_time for CI
SR-2856: adapt dispatch_timer_set_time for CI

Adjust the number of 'fast' ticks and the timeout
parameters to make the test much less likely to
be subject to spurious failures in the CI environment.

apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch