patch 8.1.2278: using "cd" with "exe" may fail

Problem:    Using "cd" with "exe" may fail.
Solution:   Use chdir() instead.
diff --git a/src/testdir/test_findfile.vim b/src/testdir/test_findfile.vim
index 5cda2ea..2e6f888 100644
--- a/src/testdir/test_findfile.vim
+++ b/src/testdir/test_findfile.vim
@@ -113,7 +113,7 @@
   call assert_match('.*/Xdir1/bar', findfile('bar', '**;', 2))
 
   bwipe!
-  exe 'cd  ' . save_dir
+  call chdir(save_dir)
   call CleanFiles()
   let &path = save_path
   let &shellslash = save_shellslash
@@ -170,7 +170,7 @@
   call assert_match('.*/Xdir1/Xdir2', finddir('Xdir2', '**;', 2))
   call assert_equal('Xdir3',          finddir('Xdir3', '**;', 1))
 
-  exe 'cd  ' . save_dir
+  call chdir(save_dir)
   call CleanFiles()
   let &path = save_path
   let &shellslash = save_shellslash