Use mock_ptr_type() to silence compiler warnings

Unit tests on 32-bit architectures were failing due to
"cast to pointer from integer of different size" warnings.
mock_ptr_type silences those by first casting to uintptr_t.

tests/unittests/config/stub_accounts.c

@@ -36,13 +36,13 @@ int  accounts_remove(const char *jid)
 
 gchar** accounts_get_list(void)
 {
-    return (gchar **)mock();
+    return mock_ptr_type(gchar **);
 }
 
 ProfAccount* accounts_get_account(const char * const name)
 {
     check_expected(name);
-    return (ProfAccount*)mock();
+    return mock_ptr_type(ProfAccount*);
 }
 
 gboolean accounts_enable(const char * const name)