patch 9.0.0626: too many delete() calls in tests

Problem:    Too many delete() calls in tests.
Solution:   Use deferred delete where possible.
diff --git a/src/testdir/test_number.vim b/src/testdir/test_number.vim
index c2cce96..e854576 100644
--- a/src/testdir/test_number.vim
+++ b/src/testdir/test_number.vim
@@ -276,7 +276,7 @@
     set number relativenumber
     hi LineNr ctermfg=red
   [CODE]
-  call writefile(lines, 'XTest_relnr')
+  call writefile(lines, 'XTest_relnr', 'D')
 
   " Check that the balloon shows up after a mouse move
   let buf = RunVimInTerminal('-S XTest_relnr', {'rows': 10, 'cols': 50})
@@ -295,7 +295,6 @@
 
   " clean up
   call StopVimInTerminal(buf)
-  call delete('XTest_relnr')
 endfunc
 
 func Test_relativenumber_callback()
@@ -313,14 +312,13 @@
 
       call timer_start(300, 'Func')
   END
-  call writefile(lines, 'Xrnu_timer')
+  call writefile(lines, 'Xrnu_timer', 'D')
 
   let buf = RunVimInTerminal('-S Xrnu_timer', #{rows: 8})
   call TermWait(buf, 310)
   call VerifyScreenDump(buf, 'Test_relativenumber_callback_1', {})
 
   call StopVimInTerminal(buf)
-  call delete('Xrnu_timer')
 endfunc
 
 " Test for displaying line numbers with 'rightleft'