Only include mobileproviderstest with ModemManager

The MobileProviders class is only included if WITH_MODEMMANAGER_SUPPORT
is true, otherwise one gets undefined references to it.

(cherry picked from commit dc43948b)

tests/CMakeLists.txt

@@ -18,7 +18,9 @@ ecm_add_test(
     LINK_LIBRARIES Qt::Test plasmanm_editor
 )
 
-ecm_add_test(
-    mobileproviderstest.cpp
-    LINK_LIBRARIES Qt::Test plasmanm_editor
-)
+if (WITH_MODEMMANAGER_SUPPORT)
+    ecm_add_test(
+        mobileproviderstest.cpp
+        LINK_LIBRARIES Qt::Test plasmanm_editor
+    )
+endif()