Make sure the temporary error log resides in a well defined location

If a test recipe does something like this:

    indir "foo.$$" => sub {
        chmod 0500, File::Spec->curdir();
        ok(run(app(["something"])));
    }

we get a problem, because we were storing the temporary stderr file in
the current directory at all times (so while inside the 'indir', we
would attemp to store it in "foo.$$").

So, change our ways to always store that temporary file in the exact
same location, defined by the environment variable RESULT_D, or
failing that TEST_D, or failing that $TOP/test.

Reviewed-by: Rich Salz <rsalz@openssl.org>