We potentially save flash space by not enabling Mbed TLS self-tests
by default. A new test config file, TESTS/configs/mbedtls.json, is
provided to enable self tests. This newly created JSON file also
enables timing in Mbed TLS so timing gets tested.
Files have been automatically imported by setting MBED_TLS_RELEASE to
mbedtls-2.24.0 in connectivity/mbedtls/tools/importer/Makefile and
running `make` in that directory.