tests: fix QCoreApplication parameters

The argument count (first parameter) is a reference, and thus must be
kept alive for the whole lifetime of the QCoreApplication instance,
as also the Qt apidocs say.

Also properly create a "string array" for the actual args, instead
of badly casting a string to that.

This fixes sporadic crashes in this test.

src/tests/TestImport.h

@@ -13,6 +13,8 @@ public:
     ~TestImport();
 
 private:
+    int argcount;
+    char *args[1]; // the size must match what 'argcount' is set to
     QCoreApplication* app;
 
 private slots: