test-loop-block: Migrate to new assertion macros and framework

While we're at it, rename to test-loop-util.c so it matches its
source file.

We also drop the root check and solely check for CAP_SYS_ADMIN since
that's sufficient to run the tests.

test/units/TEST-02-UNITTESTS.sh

@@ -24,7 +24,7 @@ if [[ -z "${TEST_MATCH_SUBTEST:-}" ]]; then
     # in QEMU to only those that can't run in a container to avoid running
     # the same tests again in a, most likely, very slow environment
     if ! systemd-detect-virt -qc && [[ "${TEST_PREFER_NSPAWN:-0}" -ne 0 ]]; then
-        TEST_MATCH_SUBTEST="test-loop-block"
+        TEST_MATCH_SUBTEST="test-loop-util"
     else
         TEST_MATCH_SUBTEST="test-*"
     fi