core: drop unused param, move taint calculation to separate file

Follow-up for 2b28dfe6e6.

I also considered moving the function to src/basic, but since it's only used by
the manager, it doesn't seem useful.

src/test/meson.build

@@ -535,7 +535,7 @@ executables += [
                 'parallel' : false,
         },
         core_test_template + {
-                'sources' : files('test-manager.c'),
+                'sources' : files('test-taint.c'),
         },
         core_test_template + {
                 'sources' : files('test-namespace.c'),

src/test/test-manager.c

@@ -1,19 +0,0 @@
-/* SPDX-License-Identifier: LGPL-2.1-or-later */
-
-#include "manager.h"
-#include "tests.h"
-
-TEST(manager_taint_string) {
-        Manager m = {};
-
-        _cleanup_free_ char *a = manager_taint_string(&m);
-        assert_se(a);
-        log_debug("taint string: '%s'", a);
-
-        if (cg_all_unified() == 0)
-                assert_se(strstr(a, "cgroupsv1"));
-        else
-                assert_se(!strstr(a, "cgroupsv1"));
-}
-
-DEFINE_TEST_MAIN(LOG_DEBUG);

src/test/test-taint.c

@@ -0,0 +1,14 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include "taint.h"
+#include "tests.h"
+
+TEST(taint_string) {
+        _cleanup_free_ char *a = taint_string();
+        assert_se(a);
+        log_debug("taint string: '%s'", a);
+
+        assert_se(!!strstr(a, "cgroupsv1") == (cg_all_unified() == 0));
+}
+
+DEFINE_TEST_MAIN(LOG_DEBUG);