core/tests: add nm_utils_get_testing() function

Code that is testable often needs special hooks to work
both for unit-tests and production.

Add a function nm_utils_get_testing() that returns whether
the code is run as part of a unit-test.

For non-testing mode, nm_utils_get_testing() will return
zero (NM_UTILS_TEST_NONE). For unit tests, the test should call
_nm_utils_set_testing() to configure tested functions.
By specifing the @flags attribute, the test can enable/disable
specific behaviors.

https://bugzilla.gnome.org/show_bug.cgi?id=701112

src/NetworkManagerUtils.h

@@ -189,4 +189,21 @@ void nm_utils_array_remove_at_indexes (GArray *array, const guint *indexes_to_de
 
 void nm_utils_setpgid (gpointer unused);
 
+typedef enum {
+	NM_UTILS_TEST_NONE                              = 0,
+
+	/* Internal flag, marking that either nm_utils_get_testing() or _nm_utils_set_testing() was called. */
+	_NM_UTILS_TEST_INITIALIZED                      = (1LL << 0),
+
+	/* Indicate that test mode is enabled in general. Explicitly calling _nm_utils_set_testing() will always set this flag. */
+	_NM_UTILS_TEST_GENERAL                          = (1LL << 1),
+
+	_NM_UTILS_TEST_LAST,
+	NM_UTILS_TEST_ALL                               = (((_NM_UTILS_TEST_LAST - 1) << 1) - 1) & ~(_NM_UTILS_TEST_INITIALIZED),
+} NMUtilsTestFlags;
+
+gboolean nm_utils_get_testing_initialized (void);
+NMUtilsTestFlags nm_utils_get_testing (void);
+void _nm_utils_set_testing (NMUtilsTestFlags flags);
+
 #endif /* __NETWORKMANAGER_UTILS_H__ */