tests: get rid of test-helper.[ch] completely

I don't think there's any particular reason to keep those functions in a separate
file.

src/shared/tests.c

@@ -15,11 +15,14 @@
 #undef basename
 
 #include "alloc-util.h"
+#include "cgroup-setup.h"
+#include "cgroup-util.h"
 #include "env-file.h"
 #include "env-util.h"
 #include "fs-util.h"
 #include "log.h"
 #include "path-util.h"
+#include "random-util.h"
 #include "strv.h"
 #include "tests.h"
 
@@ -168,3 +171,26 @@ bool can_memlock(void) {
         assert_se(munmap(p, CAN_MEMLOCK_SIZE) >= 0);
         return b;
 }
+
+int enter_cgroup_subroot(void) {
+        _cleanup_free_ char *cgroup_root = NULL, *cgroup_subroot = NULL;
+        CGroupMask supported;
+        int r;
+
+        r = cg_pid_get_path(NULL, 0, &cgroup_root);
+        if (r == -ENOMEDIUM)
+                return log_warning_errno(r, "cg_pid_get_path(NULL, 0, ...) failed: %m");
+        assert(r >= 0);
+
+        assert_se(asprintf(&cgroup_subroot, "%s/%" PRIx64, cgroup_root, random_u64()) >= 0);
+        assert_se(cg_mask_supported(&supported) >= 0);
+
+        /* If this fails, then we don't mind as the later cgroup operations will fail too, and it's fine if
+         * we handle any errors at that point. */
+
+        r = cg_create_everywhere(supported, _CGROUP_MASK_ALL, cgroup_subroot);
+        if (r < 0)
+                return r;
+
+        return cg_attach_everywhere(supported, cgroup_subroot, 0, NULL, NULL);
+}