Add iterations option to run_unit_tests

e.g. ./run_unit_tests.sh -i 100 net_test_osi.EagerReaderTest.test_large_data_multibyte

Tests are reported as failed if they fail in one or more of the iterations,
and will report a count of failures. Useful for smoking out flakey tests.

Change-Id: Ibb1c1d4c3023d59782945b8f6d9bde06bfe8c589