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.

This allows us to verify the support for Mbed TLS timing on Mbed OS.

Note: The macros MBEDTLS_TIMING_C and MBEDTLS_TIMING_ALT are not
enabled by default and need to be additionally enabled to run this
test.