Add support for --config <path-to-testconfig.json> file

[Evangelink]

Summary

Add support for specifying a different configuration file.

Background and Motivation

Currently the platform will auto-detect a [appname].testconfig.json file and load it. As part of a multi-test project, I probably want to share the same config file for all/most/multiple of my test projects. Having the ability to provide the common configuration file would be really nice.

Proposed Feature

Add support for --config <path>.

Alternative Designs

1. We could have each config file testconfig.json specifying a path to a common file to load but this is cumbersome as I need to create the config file for each of my test projects.

2. We could use an auto-lookup mechanism, similar to request Lookup test configuration from a file #3559. This seems interesting but raise the question of merging of configs.