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/shared/tests.h

@@ -2,5 +2,5 @@
 #pragma once
 
 char* setup_fake_runtime_dir(void);
-bool test_is_running_from_builddir(char **exedir);
 const char* get_testdata_dir(void);
+const char* get_catalog_dir(void);