config-parser: Don't use dbus_setenv() to test service directories

config-parser: Don't use dbus_setenv() to test service directories

We can rely on the Autotools build system to pass in some safe values
for XDG_DATA_HOME and XDG_DATA_DIRS that match DBUS_TEST_BUILDDIR.

This test will now be skipped when running test-bus manually,
or under the CMake build system. Under CMake it could be reinstated
by setting the right environment variables.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=99825
Reviewed-by: Philip Withnall <withnall@endlessm.com>
[smcv: add missing newline as requested]
[smcv: align DBUS_TEST_BUILDDIR with G_TEST_BUILDDIR]
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>