ensure visibility of test failures

In some cases, failure of an API call would result in the $res variable
being unchanged.  The test would then incorrectly pass due to a previous
test passing.
make tests conform with documented API

WWW::Postmark->send() is documented as returning "a true value" without
being specific.
add tests for further details on successful send

See [rt.cpan.org #84512].
correct small POD typo
return further details on successful send

Resolves [rt.cpan.org #84512].