runtime(syntax-tests): Adapt "runtime/syntax/testdir/vimcmd" for "src/testdir/util...

runtime(syntax-tests): Adapt "runtime/syntax/testdir/vimcmd" for "src/testdir/util/shared.vim"

Problem:  When "util/shared.vim" is sourced and GetVimCommand is called on
          behalf of the syntax test runner, the "vimcmd" file will be searched
          in the current working directory, i.e. "src/testdir", whereas the
          desired file is in "runtime/syntax/testdir".
Solution: Temporarily copy "vimcmd" between test directories.

closes: #19127

Signed-off-by: Aliaksei Budavei <0x000c70@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>