patch 8.1.0255: backup test fails when using shadow directory

Problem:    Backup test fails when using shadow directory.
Solution:   Remove check for "src".
diff --git a/src/testdir/test_backup.vim b/src/testdir/test_backup.vim
index 3187b58..0981efd 100644
--- a/src/testdir/test_backup.vim
+++ b/src/testdir/test_backup.vim
@@ -29,7 +29,7 @@
   sp *Xbackup.txt~
   call assert_equal(['line1', 'line2', 'line3'], getline(1,'$'))
   let f=expand('%')
-  call assert_match('src%testdir%Xbackup.txt\~', f)
+  call assert_match('%testdir%Xbackup.txt\~', f)
   bw!
   bw!
   call delete('Xbackup.txt')
@@ -49,7 +49,7 @@
   sp *Xbackup.txt~
   call assert_equal(['line1', 'line2', 'line3'], getline(1,'$'))
   let f=expand('%')
-  call assert_match('src%testdir%Xbackup.txt\~', f)
+  call assert_match('%testdir%Xbackup.txt\~', f)
   bw!
   bw!
   call delete('Xbackup.txt')