app-office/gnucash: Fix 2.7.4 test failures

app-office/gnucash: Fix 2.7.4 test failures

Fix a double-free bug in test-userdata-dir.c.

    Copy over common/test-core/unittest_support.py to the BUILDDIR so that
    it can be sourced by the python-bindings test.

    Set XDG_DATA_HOME so that it’s pointing to the same place as TMPDIR.

Not fixed: Locale dependent tests. /etc/locale.gen needs the
following:

    en_GB.UTF-8 UTF-8
    en_US.UTF-8 UTF-8
    fr_FR.UTF-8 UTF-8

Bug: https://bugs.gentoo.org/647596
Package-Manager: Portage-2.3.19, Repoman-2.3.6