Fix some mingw test failures.
* gettext-tools/tests/init-env.in (wabs_srcdir, wabs_top_srcdir): New variables.
(GETTEXTDATADIR): On mingw, use native Windows syntax.
* gettext-tools/tests/format-c-3: Use wabs_srcdir instead of abs_srcdir, and
wabs_top_srcdir instead of abs_top_srcdir.
* gettext-tools/tests/format-c-4: Likewise.
* gettext-tools/tests/format-c-5: Likewise.
* gettext-tools/tests/gettextpo-1: Likewise.
* gettext-tools/tests/intl-1: Likewise.
* gettext-tools/tests/intl-2: Likewise.
* gettext-tools/tests/intl-3: Likewise.
* gettext-tools/tests/intl-4: Likewise.
* gettext-tools/tests/intl-setlocale-1: Likewise.
* gettext-tools/tests/intl-setlocale-2: Likewise.
* gettext-tools/tests/intl-thread-1: Likewise.
* gettext-tools/tests/intl-thread-2: Likewise.
* gettext-tools/tests/intl-thread-3: Likewise.
* gettext-tools/tests/msgfmt-qt-1: Likewise.
* gettext-tools/tests/msgfmt-qt-2: Likewise.
* gettext-tools/tests/msginit-3: Likewise.
* gettext-tools/tests/msginit-4: Likewise.
* gettext-tools/tests/msgmerge-compendium-5: Likewise.
* gettext-tools/tests/msgmerge-compendium-6: Likewise.
* gettext-tools/tests/msgunfmt-3: Likewise.
* gettext-tools/tests/msgunfmt-tcl-1: Likewise.
* gettext-tools/tests/msguniq-1: Likewise.
* gettext-tools/tests/msguniq-2: Likewise.
* gettext-tools/tests/msguniq-3: Likewise.
* gettext-tools/tests/msguniq-4: Likewise.
* gettext-tools/tests/plural-1: Likewise.
* gettext-tools/tests/xgettext-1: Likewise.
* gettext-tools/tests/xgettext-c-1: Likewise.
* gettext-tools/tests/xgettext-c-comment-6: Likewise.
* gettext-tools/tests/xgettext-c-escape-3: Likewise.
* gettext-tools/tests/xgettext-vala-2: Likewise.
