patch 9.0.1453: typos in source code and tests

Problem:    Typos in source code and tests.
Solution:   Fi the typos. (Dominique Pellé, closes #12217)
diff --git a/src/testdir/test_mswin_event.vim b/src/testdir/test_mswin_event.vim
index e2add3b..4de0169 100644
--- a/src/testdir/test_mswin_event.vim
+++ b/src/testdir/test_mswin_event.vim
@@ -6,7 +6,7 @@
 source mouse.vim
 
 " Helper function for sending a grouped sequence of low level key presses
-" The modifer key(s) can be included as VK Key Codes in the sequence
+" The modifier key(s) can be included as VK Key Codes in the sequence
 " Keydown events will be sent, to to the end of the group, then keyup events
 " will be sent in reverse order to release the keys.
 func SendKeyGroup(keygroup)
@@ -19,7 +19,7 @@
 endfunc
 
 " Send individual key press and release events.
-" the modifers for the key press can be specified in the modifiers arg.
+" the modifiers for the key press can be specified in the modifiers arg.
 func SendKeyWithModifiers(key, modifiers)
   let args = { }
   let args.keycode = a:key
@@ -359,7 +359,7 @@
     " and when the virtual termcap maps the character
     call assert_equal(0, mod_mask, $"key = {kstr}")
 
-    " Send as a single key press with a modifers mask.
+    " Send as a single key press with a modifiers mask.
     let modifiers = 0
     let key = kcodes[0]
     for key in kcodes