patch 9.0.0611: tests delete files with a separate delete() call

Problem:    Tests delete files with a separate delete() call.
Solution:   Use deferred delete.
diff --git a/src/testdir/test_file_perm.vim b/src/testdir/test_file_perm.vim
index bb6573b..8af9532 100644
--- a/src/testdir/test_file_perm.vim
+++ b/src/testdir/test_file_perm.vim
@@ -1,26 +1,25 @@
 " Test getting and setting file permissions.
 
 func Test_file_perm()
-  call assert_equal('', getfperm('Xtest'))
-  call assert_equal(0, 'Xtest'->setfperm('r--------'))
+  call assert_equal('', getfperm('XtestPerm'))
+  call assert_equal(0, 'XtestPerm'->setfperm('r--------'))
 
-  call writefile(['one'], 'Xtest')
-  call assert_true(len('Xtest'->getfperm()) == 9)
+  call writefile(['one'], 'XtestPerm', 'D')
+  call assert_true(len('XtestPerm'->getfperm()) == 9)
 
-  call assert_equal(1, setfperm('Xtest', 'rwx------'))
+  call assert_equal(1, setfperm('XtestPerm', 'rwx------'))
   if has('win32')
-    call assert_equal('rw-rw-rw-', getfperm('Xtest'))
+    call assert_equal('rw-rw-rw-', getfperm('XtestPerm'))
   else
-    call assert_equal('rwx------', getfperm('Xtest'))
+    call assert_equal('rwx------', getfperm('XtestPerm'))
   endif
 
-  call assert_equal(1, setfperm('Xtest', 'r--r--r--'))
-  call assert_equal('r--r--r--', getfperm('Xtest'))
+  call assert_equal(1, setfperm('XtestPerm', 'r--r--r--'))
+  call assert_equal('r--r--r--', getfperm('XtestPerm'))
 
-  call assert_fails("setfperm('Xtest', '---')")
+  call assert_fails("setfperm('XtestPerm', '---')")
 
-  call assert_equal(1, setfperm('Xtest', 'rwx------'))
-  call delete('Xtest')
+  call assert_equal(1, setfperm('XtestPerm', 'rwx------'))
 
   call assert_fails("call setfperm(['Xpermfile'], 'rw-rw-rw-')", 'E730:')
   call assert_fails("call setfperm('Xpermfile', [])", 'E730:')