Use nqp::can to test for test-assertion

In some cases, specifically on the JVM, frames can be NQP code objects.
So instead of using .? dispatch, we're just checking if it can do the
'is-test-assertion' method.  Which it only can if the code actually had
the "is test-assertion` trait applied with a true value.

This should fix #3999, Christian Bartolomäus++ for spotting.

lib/Test.rakumod

@@ -771,8 +771,9 @@ sub proclaim(Bool(Mu) $cond, $desc is copy, $unescaped-prefix = '') {
         my $tester = $caller;
 
         repeat {
+            my \code := ($caller = callframe($level++)).code;
             $tester = callframe($level)  # the next one should be reported
-              if ($caller = callframe($level++)).code.?is-test-assertion;
+              if nqp::can(code,'is-test-assertion');  # must use nqp
         } until $caller.file.ends-with('.nqp');
 
         # the final place we want to report from