Simple conformance/regression tests for cmd

cmd has many features, so we will need lots of test cases.
The ones included so far are just a small sampling.

Each test consists of a .cmd file (the commands to run)
and a .out.exp file (stdout from cmd running the .cmd file).

stderr is ignored at the moment (partly because it tends
to be full of localized error messages that are hard to
compare across versions of windows and across languages).

Commands that output natural language (e.g. english)
should be redirected to nul for now, so they don't
fail on non-english developers' systems.  We may
do something nicer later.

@@ is a special keyword in the .out.exp file which
is expanded to the directory in which the tests are
run plus a backslash before comparison.

The tests should be integrated into the wine build system,
but can be built standalone by running build.sh
(you have to run "winetricks mingw" first, though
you probably want to wait until I commit a fix to winetricks,
as it currently sets the path wrong).

If WINETEST_INTERACTIVE is set, cmd_test.exe will read
tests from the current directory instead of from its
bundled resources, and will leave the logs around
for you to look at.