window: Make monitor from rect finder helper more clearly named

The function finds a suitable logical monitor given the window
rectangle; this wasn't all that clear from the name
"calculate_main_logical_monitor".

This is in preparation for finding a new logical monitor using things
other than the geometry of the window.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2554>

src/tests/test-runner.c

@@ -443,7 +443,7 @@ parse_window_size (MetaWindow *window,
   MetaRectangle logical_monitor_layout;
   int value;
 
-  logical_monitor = meta_window_calculate_main_logical_monitor (window);
+  logical_monitor = meta_window_find_monitor_from_frame_rect (window);
   g_assert_nonnull (logical_monitor);
 
   logical_monitor_layout = meta_logical_monitor_get_layout (logical_monitor);