Update tests for warnings as correctly seen post-d34aaa49

Prior to d34aaa4, the test would pass in two circumstances:
 1. There was one warning, which matched "Couldn't parse..."
 2. There were two or more warnings, the first two of which were
    identical

Case (2) was in effect, not case (1) as the code may have mislead one to
believe.  As such, the test was even more confusingly useless prior.

Update the test to make explicit that it expects the warning twice.

t/web/cf_date.t

@@ -186,7 +186,7 @@ diag 'check invalid inputs';
 
     my @warnings = $m->get_warnings;
     chomp @warnings;
-    is_deeply( [@warnings], [q{Couldn't parse date 'foodate' by Time::ParseDate}] );
+    is_deeply( [@warnings], [(q{Couldn't parse date 'foodate' by Time::ParseDate})x2] );
 }
 
 diag 'retain values when adding attachments';

t/web/cf_datetime.t

@@ -212,7 +212,7 @@ diag 'check invalid inputs';
 
     my @warnings = $m->get_warnings;
     chomp @warnings;
-    is_deeply( [@warnings], [q{Couldn't parse date 'foodate' by Time::ParseDate}] );
+    is_deeply( [@warnings], [(q{Couldn't parse date 'foodate' by Time::ParseDate})x2] );
 }
 
 diag 'retain values when adding attachments';