Implement gui_mch_mousehide to make merging from upstream easier

Add an empty implementation to remove a diff from upstream, which
constantly shows up when resolving merge conflicts. We just make the
implementation empty because we already handle mouse hide in the MacVim
process using NSCursor instead, and don't need Vim's manual handling of
it.

Also, refactor test_macvim.vim a little.

src/MacVim/gui_macvim.m

@@ -1364,6 +1364,15 @@
     ASLogInfo(@"Not implemented!");
 }
 
+    void
+gui_mch_mousehide(int hide UNUSED)
+{
+    // We don't implement this. `insertVimStateMessage` already sends this to
+    // MacVim, and we handle this in the parent process using NSCursor's
+    // `setHiddenUntilMouseMoves` instead of letting Vim have manual control
+    // over this.
+}
+
 
     void
 mch_set_mouse_shape(int shape)

src/option.c

@@ -3715,10 +3715,8 @@ did_set_modified(optset_T *args)
     char *
 did_set_mousehide(optset_T *args UNUSED)
 {
-# if !defined(FEAT_GUI_MACVIM)
     if (!p_mh)
 	gui_mch_mousehide(FALSE);
-# endif
     return NULL;
 }
 #endif

src/proto/gui_macvim.pro

@@ -77,6 +77,7 @@ void gui_mch_set_sp_color(guicolor_T color);
 void gui_mch_set_text_area_pos(int x, int y, int w, int h);
 void gui_mch_set_winpos(int x, int y);
 void gui_mch_setmouse(int x, int y);
+void gui_mch_mousehide(int hide);
 void gui_mch_settitle(char_u *title, char_u *icon);
 void gui_mch_start_blink(void);
 void gui_mch_stop_blink(int may_call_gui_update_cursor);

src/testdir/test_macvim.vim

@@ -6,16 +6,19 @@ CheckFeature gui_macvim
 " Tests for basic existence of commands and options to make sure no
 " regressions have accidentally removed them
 func Test_macvim_options_commands_exist()
+    " MacVim-specific options
     call assert_true(exists('+antialias'), 'Missing option "antialias"')
     call assert_true(exists('+blurradius'), 'Missing option "blurradius"')
     call assert_true(exists('+fullscreen'), 'Missing option "fullscreen"')
     call assert_true(exists('+fuoptions'), 'Missing option "fuoptions"')
     call assert_true(exists('+macligatures'), 'Missing option "macligatures"')
     call assert_true(exists('+macmeta'), 'Missing option "macmeta"')
     call assert_true(exists('+macthinstrokes'), 'Missing option "macthinstrokes"')
-    call assert_true(exists('+toolbariconsize'), 'Missing option "toolbariconsize"')
     call assert_true(exists('+transparency'), 'Missing option "transparency"')
 
+    " Other GUI options we care about
+    call assert_true(exists('+toolbariconsize'), 'Missing option "toolbariconsize"')
+
     call assert_true(exists(':macaction'), 'Missing command "macaction"')
     call assert_true(exists(':macmenu'), 'Missing command "macmenu"')