patch 9.1.2076: tests: MinGW test fails midway and stops

patch 9.1.2076: tests: MinGW test fails midway and stops

Problem:  tests: When running the MinGW test, an error occurs after
          generating opt_test.vim.
Solution: Rewrite the error detection in cmd.exe syntax (Muraoka Taro).

The recipe section of src/testdir/Make_ming.mak must be written in
cmd.exe syntax.  This is because "SHELL = cmd.exe" is specified at the
beginning.  However, the error detection in the opt_test.vim recipe is
written in UNXI syntax, which caused the error.

closes: #19146

Signed-off-by: Muraoka Taro <koron.kaoriya@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>