Add a golden test framework

[gallais]

We shouldn't be able to introduce bugs like I did when refactoring IO
(fixed in #1451) or the readMaybe one discussed in agda/agda/issues/5288.

One piece of the puzzle is to have a golden test framework and have
a lot of tests involving the standard library's more "stringly typed"
functions.

A possible source of inspiration:
https://github.com/idris-lang/Idris2/blob/master/libs/test/Test/Golden.idr

[MatthewDaggitt]

This was completed in #1518