Increase randomness of context switches for test

Result of TrueParallelTest is inherently nondeterministic, on systems
with high number of processors, the default number 5 of concurrent running
threads is too low to make the test pass. On my 8 cores system, the test
can rarely pass. This patch changes the number of threads to 4 times of
available processors to make the test more likely to pass. But still
the result of this test is nondeterministic.
Increase scheduling randomness

This patch increases scheduling randomness to make TureParallelTest more likely to pass.
Merge pull request #362 from sunxiaoguang/nondeterministic_parallel_test

The result of TrueParallelTest is nondeterministic