patch 8.2.3903: "gM" does not count tabs as expected

Problem:    "gM" does not count tabs as expected.
Solution:   Use linetabsize() instead of mb_string2cells(). (closes #9409)
diff --git a/src/testdir/test_normal.vim b/src/testdir/test_normal.vim
index 6458a1b..9794961 100644
--- a/src/testdir/test_normal.vim
+++ b/src/testdir/test_normal.vim
@@ -2580,7 +2580,15 @@
   call assert_equal(87, col('.'))
   call assert_equal('E', getreg(0))
 
+  " Test for gM with Tab characters
+  call setline('.', "\ta\tb\tc\td\te\tf")
+  norm! gMyl
+  call assert_equal(6, col('.'))
+  call assert_equal("c", getreg(0))
+
   " Test for g Ctrl-G
+  call setline('.', lineC)
+  norm! 60gMyl
   set ff=unix
   let a=execute(":norm! g\<c-g>")
   call assert_match('Col 87 of 144; Line 2 of 2; Word 1 of 1; Byte 88 of 146', a)