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.

src/testdir/test_functions.vim

@@ -1326,9 +1326,8 @@ func Test_filewritable()
 
   call assert_equal(0, filewritable('doesnotexist'))
 
-  call mkdir('Xwritedir')
+  call mkdir('Xwritedir', 'D')
   call assert_equal(2, filewritable('Xwritedir'))
-  call delete('Xwritedir', 'd')
 
   call delete('Xfilewritable')
   bw!
@@ -1661,7 +1660,7 @@ func Test_setbufvar_keep_window_title()
       let g:buf = bufadd('Xb.txt')
       inoremap <F2> <C-R>=setbufvar(g:buf, '&autoindent', 1) ?? ''<CR>
   END
-  call writefile(lines, 'Xsetbufvar')
+  call writefile(lines, 'Xsetbufvar', 'D')
   let buf = RunVimInTerminal('-S Xsetbufvar', {})
   call WaitForAssert({-> assert_match('Xa.txt', term_gettitle(buf))}, 1000)
 
@@ -1672,7 +1671,6 @@ func Test_setbufvar_keep_window_title()
   call assert_match('Xa.txt', term_gettitle(buf))
 
   call StopVimInTerminal(buf)
-  call delete('Xsetbufvar')
 endfunc
 
 func Test_redo_in_nested_functions()
@@ -1938,19 +1936,18 @@ endfunc
 func Test_func_range_with_edit()
   " Define a function that edits another buffer, then call it with a range that
   " is invalid in that buffer.
-  call writefile(['just one line'], 'Xfuncrange2')
+  call writefile(['just one line'], 'Xfuncrange2', 'D')
   new
   eval 10->range()->setline(1)
   write Xfuncrange1
   call assert_fails('5,8call EditAnotherFile()', 'E16:')
 
   call delete('Xfuncrange1')
-  call delete('Xfuncrange2')
   bwipe!
 endfunc
 
 func Test_func_exists_on_reload()
-  call writefile(['func ExistingFunction()', 'echo "yes"', 'endfunc'], 'Xfuncexists')
+  call writefile(['func ExistingFunction()', 'echo "yes"', 'endfunc'], 'Xfuncexists', 'D')
   call assert_equal(0, exists('*ExistingFunction'))
   source Xfuncexists
   call assert_equal(1, '*ExistingFunction'->exists())
@@ -1959,7 +1956,7 @@ func Test_func_exists_on_reload()
   call assert_equal(1, exists('*ExistingFunction'))
 
   " But redefining in another script is not OK.
-  call writefile(['func ExistingFunction()', 'echo "yes"', 'endfunc'], 'Xfuncexists2')
+  call writefile(['func ExistingFunction()', 'echo "yes"', 'endfunc'], 'Xfuncexists2', 'D')
   call assert_fails('source Xfuncexists2', 'E122:')
 
   " Defining a new function from the cmdline should fail if the function is
@@ -1975,8 +1972,6 @@ func Test_func_exists_on_reload()
   call assert_fails('source Xfuncexists', 'E122:')
   call assert_equal(1, exists('*ExistingFunction'))
 
-  call delete('Xfuncexists2')
-  call delete('Xfuncexists')
   delfunc ExistingFunction
 endfunc
 
@@ -2063,7 +2058,7 @@ func Test_platform_name()
 endfunc
 
 func Test_readdir()
-  call mkdir('Xreaddir')
+  call mkdir('Xreaddir', 'R')
   call writefile([], 'Xreaddir/foo.txt')
   call writefile([], 'Xreaddir/bar.txt')
   call mkdir('Xreaddir/dir')
@@ -2092,12 +2087,10 @@ func Test_readdir()
   " Nested readdir() must not crash
   let files = readdir('Xreaddir', 'readdir("Xreaddir", "1") != []')
   call sort(files)->assert_equal(['bar.txt', 'dir', 'foo.txt'])
-
-  eval 'Xreaddir'->delete('rf')
 endfunc
 
 func Test_readdirex()
-  call mkdir('Xexdir')
+  call mkdir('Xexdir', 'R')
   call writefile(['foo'], 'Xexdir/foo.txt')
   call writefile(['barbar'], 'Xexdir/bar.txt')
   call mkdir('Xexdir/dir')
@@ -2144,7 +2137,6 @@ func Test_readdirex()
     call sort(files)->assert_equal(
         \ ['bar.txt_file', 'dir_dir', 'foo.txt_file', 'link_link'])
   endif
-  eval 'Xexdir'->delete('rf')
 
   call assert_fails('call readdirex("doesnotexist")', 'E484:')
 endfunc
@@ -2156,7 +2148,7 @@ func Test_readdirex_sort()
     throw 'Skipped: Test_readdirex_sort on systems that do not allow this using the default filesystem'
   endif
   let _collate = v:collate
-  call mkdir('Xsortdir2')
+  call mkdir('Xsortdir2', 'R')
   call writefile(['1'], 'Xsortdir2/README.txt')
   call writefile(['2'], 'Xsortdir2/Readme.txt')
   call writefile(['3'], 'Xsortdir2/readme.txt')
@@ -2198,14 +2190,13 @@ func Test_readdirex_sort()
 
   finally
     exe 'lang collate' collate
-    eval 'Xsortdir2'->delete('rf')
   endtry
 endfunc
 
 func Test_readdir_sort()
   " some more cases for testing sorting for readdirex
   let dir = 'Xsortdir3'
-  call mkdir(dir)
+  call mkdir(dir, 'R')
   call writefile(['1'], dir .. '/README.txt')
   call writefile(['2'], dir .. '/Readm.txt')
   call writefile(['3'], dir .. '/read.txt')
@@ -2247,8 +2238,6 @@ func Test_readdir_sort()
 
   " Cleanup
   exe "lang collate" collate
-
-  eval dir->delete('rf')
 endfunc
 
 func Test_delete_rf()