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'.

scripts/vim-patch.sh

@@ -226,6 +226,10 @@ preprocess_patch() {
 ' +w +q "$file"
 
   # Remove some *.po files. #5622
+  local na_po='sjiscorr\.c\|ja\.sjis\.po\|ko\.po\|pl\.cp1250\.po\|pl\.po\|ru\.cp1251\.po\|uk\.cp1251\.po\|zh_CN\.cp936\.po\|zh_CN\.po\|zh_TW\.po'
+  2>/dev/null $nvim --cmd 'set dir=/tmp' +'g@^diff --git a/src/po/\<\%('${na_po}'\)\>@norm! d/\v(^diff)|%$
+' +w +q "$file"
+
   # Remove vimrc_example.vim
   local na_vimrcexample='vimrc_example\.vim'
   2>/dev/null $nvim --cmd 'set dir=/tmp' +'g@^diff --git a/runtime/\<\%('${na_vimrcexample}'\)\>@norm! d/\v(^diff)|%$