Unify mail related tests for *nix and Windows

Hi all! Currently our mail related tests (in ext/standard/tests/mail/ and maybe other locations) are split for *nix and Windows (if there are even Windows versions). The basic difference is that the *nix variants set the INI directive sendmail_path to just write the email to disk (some variations exist), while the Windows tests use ext/imap. The latter tests are way more verbose, and such duplicated tests are generally a pain point. Therefore I suggest to unify these tests. Basically, setting the sendmail_path works for Windows as well; we only have to offer a binary that writes the mail to disk (tee and non-binaries don't appear to work). A rudimentary implementation is trivial[1]. The then missing part would be to actually make the ini setting portable, which might be done by introducing a new placeholder for PHPT INI sections, similar to {TMP} and {PWD}. The new placeholder would likely have to accept a filename, so it could be something like {MAIL:<filename>}. Any ideas and suggestions for improvements, or potential caveats welcome! [1] <https://gist.github.com/cmb69/40c8c3f40795d9032aec49d13e452fa5> -- Christoph M. Becker