unit-tests: Add an option to run only a subset of all test suites

The TESTS_SUITES environment variable can contain a comma separated list
of names of test suites to run.
unit-tests: Add an option to increase the verbosity when running tests

The TESTS_VERBOSITY option takes an integer from -1 to 4 that sets the
default debug level.
crypto-tester: Don't fail if key size is not supported

The Blowfish and Twofish implementations provided by the gcrypt plugin
only support specific key lengths, which we don't know when testing
against vectors (either during unit tests or during algorithm
registration).  The on_create test with a specific key length will be
skipped anyway, so there is no point in treating this failure differently.
travis: Use parallel build