various improvements to the Synchronized tests

various improvements to the Synchronized tests

Summary:
Add a runParallel() helper function, to clean up logic that was copy-and-pasted
through most of the test functions.  Additionally, clean up the tests to avoid
unnecessary sleeps.  Also fix backwards arguments to EXPECT_EQ() calls--gtest
assumes the first argument is the expected value, and the second argument is
the value being checked.

Reviewed By: yfeldblum

Differential Revision: D3521565

fbshipit-source-id: e4f007d52c114080cff1fd7a0a407fba39fa8b0e