fix flaky tests

[thinkingfish]

Add a mock timer for tests that rely on timing, such as pipe tcp timewheel, these occasionally fail due to timing issues.

[brayniac]

Seeing periodic flakey tests on the linux gcc builds.

	  4 - check_pipe (Failed)
	  5 - check_tcp (Failed)
Errors while running CTest
Makefile:85: recipe for target 'test' failed
make: *** [test] Error 8
_build/test/channel/tcp/tcp.log:/home/travis/build/twitter/ccommon/test/channel/tcp/check_tcp.c:335:F:tcp test:test_nonblocking:0: Assertion 'duration_us(&duration) >= 500000' failed: duration_us(&duration) == 499977, 500000 == 500000
_build/test/channel/pipe/pipe.log:/home/travis/build/twitter/ccommon/test/channel/pipe/check_pipe.c:129:F:pipe test:test_read_blocking:0: Assertion 'duration_us(&duration) >= 500000' failed: duration_us(&duration) == 499959, 500000 == 500000
Build failure```