patch 8.2.1340: some tests fail on Cirrus CI and/or with FreeBSD

Problem:    Some tests fail on Cirrus CI and/or with FreeBSD.
Solution:   Make 'backupskip' empty. Do not run tests as root. Check for
            directory when using viminfo. (Ozaki Kiichi, closes #6596)
diff --git a/src/testdir/test_backup.vim b/src/testdir/test_backup.vim
index 0d3e79c..4c7abe8 100644
--- a/src/testdir/test_backup.vim
+++ b/src/testdir/test_backup.vim
@@ -19,6 +19,22 @@
   call delete('Xbackup.txt~')
 endfunc
 
+func Test_backup_backupskip()
+  set backup backupdir=. backupskip=*.txt
+  new
+  call setline(1, ['line1', 'line2'])
+  :f Xbackup.txt
+  :w! Xbackup.txt
+  " backup file is only created after
+  " writing a second time (before overwriting)
+  :w! Xbackup.txt
+  call assert_false(filereadable('Xbackup.txt~'))
+  bw!
+  set backup&vim backupdir&vim backupskip&vim
+  call delete('Xbackup.txt')
+  call delete('Xbackup.txt~')
+endfunc
+
 func Test_backup2()
   set backup backupdir=.// backupskip=
   new
@@ -30,7 +46,7 @@
   :w! Xbackup.txt
   sp *Xbackup.txt~
   call assert_equal(['line1', 'line2', 'line3'], getline(1,'$'))
-  let f=expand('%')
+  let f = expand('%')
   call assert_match('%testdir%Xbackup.txt\~', f)
   bw!
   bw!
@@ -50,7 +66,7 @@
   :w! Xbackup.txt
   sp *Xbackup.txt~
   call assert_equal(['line1', 'line2', 'line3'], getline(1,'$'))
-  let f=expand('%')
+  let f = expand('%')
   call assert_match('%testdir%Xbackup.txt\~', f)
   bw!
   bw!
@@ -61,14 +77,11 @@
 
 " Test for using a non-existing directory as a backup directory
 func Test_non_existing_backupdir()
-  CheckNotBSD
-  let save_backup = &backupdir
-  set backupdir=./non_existing_dir
+  set backupdir=./non_existing_dir backupskip=
   call writefile(['line1'], 'Xfile')
   new Xfile
-  " TODO: write doesn't fail in Cirrus FreeBSD CI test
   call assert_fails('write', 'E510:')
-  let &backupdir = save_backup
+  set backupdir&vim backupskip&vim
   call delete('Xfile')
 endfunc