Add environment variable to control the default timeout in sub_test

CHPL_TEST_TIMEOUT will set the default timeout that is used. Using this
lets you increase the timeout without reducing number of trails down to
one.
Merge pull request #643 from Kyle-B/subtest-timeout-improvements

Add environment variable to control the default timeout in sub_test

CHPL_TEST_TIMEOUT will set the default timeout that is used. Using this
lets you increase the timeout without reducing number of trails down to
one.