ExpectErrorIs checks whether got has an 'Is' relation to want (or, if want is nil, whether got is non-nil).

TestFilesOfExt is a scaffold for running a testing payload on every file in a directory dir that has extension ext.

TestJSONRoundTrip tests that encoding want to TOML then decoding it produces a value that is deep-equal to want.

TestTomlRoundTrip tests that encoding want to TOML then decoding it produces a value that is deep-equal to want.