all/tests: split core part out of "nm-test-utils.h"

A large part of "nm-test-utils.h" is only relevant for tests inside "src/"
directory, as they are helpers related to NetworkManager core part.

Split this part out of "nm-test-utils.h" header.

src/dhcp-manager/tests/test-dhcp-dhclient.c

@@ -31,7 +31,7 @@
 #include "nm-ip4-config.h"
 #include "nm-platform.h"
 
-#include "nm-test-utils.h"
+#include "nm-test-utils-core.h"
 
 #define DEBUG 1
 

src/dhcp-manager/tests/test-dhcp-utils.c

@@ -28,7 +28,7 @@
 #include "nm-dhcp-utils.h"
 #include "nm-platform.h"
 
-#include "nm-test-utils.h"
+#include "nm-test-utils-core.h"
 
 typedef struct {
 	const char *name;