runtime(syntax-tests): Improve parts of "runtest.vim"

runtime(syntax-tests): Improve parts of "runtest.vim"

* Accommodate the calling of "EraseLineAndReturnCarriage()"
  to not interfere with the "skipped" and "failed" reports.

* Create the "failed" directory, if unavailable, without
  relying on "VerifyScreenDump()" to do it so that reporting
  with "Xtestscript#s:AssertCursorForwardProgress()" can be
  uniformly attempted.

* Make an only list copy of the "Xtestscript" contents and
  share it with every syntax test.

* Narrow the scope of the "filetype" and "failed_root" local
  variables.

closes: #16789

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