Add CHPL_TEST_UNIQUIFY_EXE allowing parallel runs of sub_test

If CHPL_TEST_UNIQUIFY_EXE is set, the test has the PID of the sub_test
process appended to its executable name. This allows you to run multiple
start_tests over the same set of tests in parallel.

This feature is disabled by default and may fail with tests using
.prediffs and similar features of the test system.
Merge pull request #661 from Kyle-B/kb-test-uniquify-exe

Add CHPL_TEST_UNIQUIFY_EXE allowing parallel runs of sub_test

If CHPL_TEST_UNIQUIFY_EXE is set, the test has the PID of the sub_test
process appended to its executable name. This allows you to run multiple
start_tests over the same set of tests in parallel.

This feature is disabled by default and may fail with tests using
.prediffs and similar features of the test system.