Added readme for tests folder

src/frontend/test/README.md

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+# Test folder
+This folder contains stubs and/or copies of functionality used within the system. It is felt that (at this time), this is a "necessary" thing for unit testing - this is here because Jest (the JS unit testing framework) uses Babel behind the scenes; the `/src/frontend/lib` directory is ignored by Babel - when this ignore is removed (in order for components to build properly on testing) the JS _does not build_ - as such, the stubs and copies were created with the relevant mapping being put in place within the Jest config.
+This will change once unit testing of components (i.e. regression testing) is implemented and the config will be updated as required to use Jest environments. It was felt that at this point, rather than edit files that are integral to the running of the site, copies were the safer option.