patch 9.1.1163: $MYVIMDIR is set too late

Problem:  $MYVIMDIR is set too late and not available while sourcing
          runtime files (Maxim Kim, after v9.1.1159)
Solution: Also set it when $MYVIMRC file is found

fixes: #16764
closes: #16767

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_xdg.vim b/src/testdir/test_xdg.vim
index 15f82e2..b7abd04 100644
--- a/src/testdir/test_xdg.vim
+++ b/src/testdir/test_xdg.vim
@@ -21,7 +21,7 @@
   let rc = s:get_rcs()
   let before =<< trim CODE
     call writefile([expand('$MYVIMRC')], "XMY_VIMRC")
-    call writefile([expand('$MYVIMRCDIR')], "XMY_VIMDIR")
+    call writefile([expand('$MYVIMDIR')], "XMY_VIMDIR")
     quit!
   CODE
   call RunVim(before, [], "")
@@ -58,21 +58,30 @@
   " g:rc_one|two|three|four is to verify, that the other
   " init files are not sourced
   " g:rc is to verify which rc file has been loaded.
+  " g:rc_vimdir is to verify $MYVIMDIR is set and valid
   let file1 =<< trim CODE
     let g:rc_one = 'one'
     let g:rc = '.vimrc'
+    let g:rc_vimdir = expand('~/.vim/')
+    call assert_equal(g:rc_vimdir, $MYVIMDIR)
   CODE
   let file2 =<< trim CODE
     let g:rc_two = 'two'
     let g:rc = '.vim/vimrc'
+    let g:rc_vimdir = expand('~/.vim/')
+    call assert_equal(g:rc_vimdir, $MYVIMDIR)
   CODE
   let file3 =<< trim CODE
     let g:rc_three = 'three'
     let g:rc = '.config/vim/vimrc'
+    let g:rc_vimdir = expand('~/.config/vim/')
+    call assert_equal(g:rc_vimdir, $MYVIMDIR)
   CODE
   let file4 =<< trim CODE
     let g:rc_four = 'four'
     let g:rc = 'xdg/vim/vimrc'
+    let g:rc_vimdir = expand('~/xdg/vim/')
+    call assert_equal(g:rc_vimdir, $MYVIMDIR)
   CODE
   call writefile(file1, rc1)
   call writefile(file2, rc2)
@@ -87,7 +96,7 @@
     call assert_match('XfakeHOME/\.vimrc', $MYVIMRC)
     call assert_match('XfakeHOME/.vim/', $MYVIMDIR)
     call filter(g:, {idx, _ -> idx =~ '^rc'})
-    call assert_equal(#{rc_one: 'one', rc: '.vimrc'}, g:)
+    call assert_equal(#{rc_one: 'one', rc: '.vimrc', rc_vimdir: $MYVIMDIR}, g:)
     call assert_match('XfakeHOME/\.vim/view', &viewdir)
     call writefile(v:errors, 'Xresult')
     quit
@@ -103,7 +112,7 @@
     call assert_match('XfakeHOME/\.vim/vimrc', $MYVIMRC)
     call assert_match('XfakeHOME/\.vim/', $MYVIMDIR)
     call filter(g:, {idx, _ -> idx =~ '^rc'})
-    call assert_equal(#{rc_two: 'two', rc: '.vim/vimrc'}, g:)
+    call assert_equal(#{rc_two: 'two', rc: '.vim/vimrc', rc_vimdir: $MYVIMDIR}, g:)
     call assert_match('XfakeHOME/\.vim/view', &viewdir)
     call writefile(v:errors, 'Xresult')
     quit
@@ -123,7 +132,7 @@
     call assert_match('XfakeHOME/\.config/vim/vimrc', $MYVIMRC, msg)
     call assert_match('XfakeHOME/\.config/vim/', $MYVIMDIR, msg)
     call filter(g:, {idx, _ -> idx =~ '^rc'})
-    call assert_equal(#{rc_three: 'three', rc: '.config/vim/vimrc'}, g:)
+    call assert_equal(#{rc_three: 'three', rc: '.config/vim/vimrc', rc_vimdir: $MYVIMDIR}, g:)
     call assert_match('XfakeHOME/\.config/vim/view', &viewdir)
     call writefile(v:errors, 'Xresult')
     quit
@@ -141,7 +150,7 @@
     call assert_match('XfakeHOME/xdg/vim/vimrc', $MYVIMRC, msg)
     call assert_match('XfakeHOME/xdg/vim/', $MYVIMDIR, msg)
     call filter(g:, {idx, _ -> idx =~ '^rc'})
-    call assert_equal(#{rc_four: 'four', rc: 'xdg/vim/vimrc'}, g:)
+    call assert_equal(#{rc_four: 'four', rc: 'xdg/vim/vimrc', rc_vimdir: $MYVIMDIR}, g:)
     call assert_match('XfakeHOME/xdg/vim/view, &viewdir)
     call writefile(v:errors, 'Xresult')
     quit