Rewrite "testPerm" helper function used for testing of internal
runtime code. It's been renamed to "unitTest" and provides API that
is extensible in the future by accepting optional "UnitTestOptions"
argument. "test" helper was also removed and replaced by
overloaded version of "unitTest" that takes only function argument.
"UnitTestOptions" currently supports "perms" and "skip"
options, where former works exactly as first argument to "testPerm"
did, while the latter allows to conditionally skip tests.
Worth noting due to implementation of the Headers class the contents of headersMap have lowercase keys, although this matches the specification as header keys are case agnostic it does seem to not match behaviour of other implementations in other languages I have seen, would require some rewriting of Headers.ts
