t1800: correct test to handle Cygwin

t1800: correct test to handle Cygwin

On Cygwin, when failing to spawn a process using start_command, Git
outputs the same error as on Linux systems, rather than using the
GIT_WINDOWS_NATIVE-specific error output.  The WINDOWS test prerequisite
is set in both Cygwin and native Windows environments, which means it's
not appropriate to use to anticipate the error output from
start_command.  Instead, use the MINGW test prerequisite, which is only
set for Git in native Windows environments, and not for Cygwin.

Signed-off-by: Adam Dinwoodie <redacted>
Helped-by: Đoàn Trần Công Danh <redacted>
Signed-off-by: Junio C Hamano <redacted>