Prevent reverseRoute from searching closures

[MGatner]

Description
reverseRoute searches loaded routes for a string match, but will throw an exception on unmatched routes when it tries to trim any routes whose definitions are closures. This ignores non-string destinations in the secondary search phase.
Fix for #1953

Checklist:

• Securely signed commits
• Component(s) with PHPdocs
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[MGatner]

I believe this is still relevant, and would be cause testReverseRoutingReturnsFalseWithNoMatch() to fail except that it isn't loading default routes from system/Config/Routes.php which include Closures. I'm adding a test to demonstrate the failure.