When using the `mbed test` command to build and run tests, some
targets have additional configurations/overrides defined in
`tools/test_configs/`:
* `target_configs.json` lists which targets support which configs.
* `config_paths.json` maps the name of each config to the JSON file to
use.
By default, only `default_test_configuration` from
`target_configs.json` gets used when building and running tests.
Others listed in `test_configuration` need to be switched via
`--test-config <NAME>`.
This commit enables Experimental API in the default configurations of
K64F and K66F in order to test Mbed OS PSA. Any existing configs are
kept, which is why `HeapBlockDeviceAndEthernetAndExperimental.json` is
created for K64F.
Lingkai Dong