test: move oldtests to test directory (#22536)

The new oldtest directory is in test/old/testdir. The reason for this is
that many tests have hardcoded the parent directory name to be
'testdir'.

test/old/testdir/test_nested_function.vim

@@ -0,0 +1,63 @@
+"Tests for nested functions
+"
+func NestedFunc()
+  func! Func1()
+    let g:text .= 'Func1 '
+  endfunc
+  call Func1()
+  func! s:func2()
+    let g:text .= 's:func2 '
+  endfunc
+  call s:func2()
+  func! s:_func3()
+    let g:text .= 's:_func3 '
+  endfunc
+  call s:_func3()
+  let fn = 'Func4'
+  func! {fn}()
+    let g:text .= 'Func4 '
+  endfunc
+  call {fn}()
+  let fn = 'func5'
+  func! s:{fn}()
+    let g:text .= 's:func5'
+  endfunc
+  call s:{fn}()
+endfunc
+
+func Test_nested_functions()
+  let g:text = ''
+  call NestedFunc()
+  call assert_equal('Func1 s:func2 s:_func3 Func4 s:func5', g:text)
+endfunction
+
+func Test_nested_argument()
+  func g:X()
+    let g:Y = function('sort')
+  endfunc
+  let g:Y = function('sort')
+  echo g:Y([], g:X())
+  delfunc g:X
+  unlet g:Y
+endfunc
+
+func Recurse(count)
+  if a:count > 0
+    call Recurse(a:count - 1)
+  endif
+endfunc
+
+func Test_max_nesting()
+  let call_depth_here = 2
+  let ex_depth_here = 5
+  set mfd&
+
+  call Recurse(99 - call_depth_here)
+  call assert_fails('call Recurse(' . (100 - call_depth_here) . ')', 'E132:')
+
+  set mfd=210
+  call Recurse(209 - ex_depth_here)
+  call assert_fails('call Recurse(' . (210 - ex_depth_here) . ')', 'E169:')
+
+  set mfd&
+endfunc