Bug 1719758 - Test result values before unwrapping in TestPluralRules…

… r=platform-i18n-reviewers,dminor

Modifies TestPluralRules.cpp to assert that all result values are okay
before unwrapping them. Previously the test suite would crash if an
error result was unwrapped. Now it will fail the test and report the
failure.

Differential Revision: https://phabricator.services.mozilla.com/D119465