test: use ${builddir}/systemd-runtest.env for $SYSTEMD_CATALOG_DIR

This makes it so that tests no longer need to know the absolute paths to the
source and build dirs, instead using the systemd-runtest.env file to get these
paths when running from the build tree.

Confirmed that test-catalog works on `ninja test`, when called standalone and
also when the environment file is not present, in which case it will use the
installed location under /usr/lib/systemd/catalog.

The location can now also be overridden for this test by setting the
$SYSTEMD_CATALOG_DIR environment variable.

src/test/meson.build

@@ -756,8 +756,7 @@ tests += [
           libshared],
          [threads,
           libxz,
-          liblz4],
-         '', '', '-DCATALOG_DIR="@0@"'.format(catalogdir)],
+          liblz4]],
 
         [['src/journal/test-compress.c'],
          [libjournal_core,