tools: Reorder test configuration priority

[theotherjimmy]

Description

The mbed test subcommand Takes two arguments, --app-config and
--test-config. The test config parameter takes priority over the
app config parameter. Where things get weird is after that:
The current order (before this PR):

• --test-config
• --app-config
• test config (as defined in test_config)
• mbed_app.json in root.

That's probably because we did not have a good way to do what this PR
does:

• --test-config
• --app-config
• mbed_app.json in root.
• test config (as defined in test_config):

This gives the user more control and matches the documented behavior in
the arguments

Pull request type

[x] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1931
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6805/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Test : SUCCESS

Build number : 1753
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6805/1753

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1577
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6805/