Reorder test configs
Reorder test configs

Old order:
 * --test-config
 * --app-config
 * test config (as defined in test_config)
 * `mbed_app.json` in root.

New order:
 * --test-config
 * --app-config
 * `mbed_app.json` in root.
 * test config (as defined in test_config)~

This matches the documentation and user expectation

kjbracey-arm/mbed,c1728p9/mbed-os,andcor02/mbed-os,mbedmicro/mbed,betzw/mbed-os,kjbracey-arm/mbed,c1728p9/mbed-os,andcor02/mbed-os,mbedmicro/mbed,mbedmicro/mbed,betzw/mbed-os,c1728p9/mbed-os,mbedmicro/mbed,andcor02/mbed-os,betzw/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed,kjbracey-arm/mbed,c1728p9/mbed-os,andcor02/mbed-os,betzw/mbed-os,c1728p9/mbed-os,betzw/mbed-os,c1728p9/mbed-os,mbedmicro/mbed,betzw/mbed-os,andcor02/mbed-os