Add documentation & guides about testing

[barrettj12]

#1518 added a new golden testing framework to the standard library. It would be useful to extend the tests to cover more modules. However, at present, there is no documentation or guide on how to create new tests. We could have a TESTING.md file, which, analogously to HACKING.md, guides contributors on how to create test cases for new functions they write.

[gallais]

In Idris, we have templates for the tests. It may be appropriate to add them here too.
The advantage of templates is that we can make them part of the test suite and thus make
sure they keep building if we update the lib!