Stop running semantic3 for unittests in non-root modules

This is important due to avoid out-of-memory error with `-unittest -inline`. Currently front-end inlining requires running semantic3 for all imported modules to analyze expanded function body code. By this change, we can remove the Phobos unittest analyzing cost in user application compilation.

src/func.c

@@ -5152,10 +5152,14 @@ void UnitTestDeclaration::semantic(Scope *sc)
     protection = sc->protection;
 
     if (scope)
-    {   sc = scope;
+    {
+        sc = scope;
         scope = NULL;
     }
 
+    if (!isInstantiated() && inNonRoot())
+        return;
+
     if (global.params.useUnitTests)
     {
         if (!type)