Instead, make it so that WIREPLUMBER_*_DIR environment variables can
contain a list of directories to look into. This is safer and,
as a bonus, allows for more control over the lookup directories.
Using the G_TEST_SRCDIR variable can cause problems for tests of other
projects that use libwireplumber and may also lead to unexpected
behavior by not being obvious that this causes wireplumber to skip
looking in its standard directories...
This also brings back WIREPLUMBER_CONFIG_DIR, which is going to be
needed again for the upcoming WpConf changes.
