Replace isa_ok with isa-ok
The output of the zip operator `Z` used to be flattened implicitly. This is no longer the case, thus to reinstate the previous behaviour expected in the context tests, the zipped lists need to be explicitly flattened before their values are passed into the relevant for loop. This change makes the `context.t` tests pass.
Previously (before 2013.05) Rakudo returned the empty string from regexps which didn't match. From Rakudo 2013.05 onwards, regexps not matching a value return Nil, thus this change updates the test suite accordingly, and these tests now pass with the current Rakudo.