Fix upstream merge CI failure with 'winaltkeys'

Make sure to block testing 'winaltkeys' only if the option is supported.

Also, while we are at it, add a test to make sure MacVim-specific
options have proper error checking to prevent bad merge conflict
resolution from accidentally removing said logic.

src/testdir/test_macvim.vim

@@ -5,7 +5,7 @@ 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()
+func Test_macvim_options_commands_exist()
     call assert_true(exists('+antialias'), 'Missing option "antialias"')
     call assert_true(exists('+blurradius'), 'Missing option "blurradius"')
     call assert_true(exists('+fullscreen'), 'Missing option "fullscreen"')
@@ -62,3 +62,12 @@ func Test_macvim_mappings()
     call feedkeys("\<ForceClick>", "xt")
     call assert_equal(5, g:marker_value)
 endfunc
+
+" Test that setting invalid values with properly throw invalid argument errors
+func Test_macvim_invalid_options()
+    call assert_fails("let &blur=-1", 'E474:')
+    call assert_fails("let &transparency=-1", 'E474:')
+    call assert_fails("let &transparency=101", 'E474:')
+
+    call assert_fails("let &fuoptions='abcdef'", 'E474:')
+endfunc

src/testdir/test_options.vim

@@ -1656,7 +1656,7 @@ func Test_string_option_revert_on_failure()
     call add(optlist, ['varsofttabstop', '12', 'a123'])
     call add(optlist, ['vartabstop', '4,20', '4,'])
   endif
-  if has('gui')
+  if has('gui') && has('+winaltkeys')
     call add(optlist, ['winaltkeys', 'no', 'a123'])
   endif
   for opt in optlist