lang: If the test fails earlier than expected, exit early

If a test failed in stage 2 (fail2) instead of an expected fail in stage
3 (fail3) then it would continue running, which was an undefined
behaviour in our API. IOW we should not run Unify if SetScope failed.
This patch adds these additional checks to ensure our tests are more
robust.

lang/structs.go

@@ -3143,6 +3143,10 @@ func (obj *StmtInclude) Unify() ([]interfaces.Invariant, error) {
 	if obj.Name == "" {
 		return nil, fmt.Errorf("missing include name")
 	}
+	if obj.class == nil {
+		// possible programming error
+		return nil, fmt.Errorf("include doesn't contain a class pointer yet")
+	}
 
 	// is it even possible for the signatures to match?
 	if len(obj.class.Args) != len(obj.Args) {