test: Add an experimental API config

It's important we still continue to run `mbed test` tests for APIs
considered experimental. Add a new test config which enables all
experimental APIs for test purposes.

For example, to run mbed test including experimental APIs on K64F:
    mbed test -m K64F -t GCC_ARM --app-config TESTS/configs/experimental.json

TESTS/configs/experimental.json

@@ -0,0 +1,9 @@
+{
+    "target_overrides": {
+        "*": {
+             "target.features_add": [
+                 "EXPERIMENTAL_API"
+             ]
+        }
+    }
+}