tests: remove the DNS stand-ins from the watcher and resolver tests (closes #159)

The watcher tests used a stand-in resolver and the resolver timeout test
a stand-in DNS client, against the rule that DNS is never mocked. The
watcher tests now run the real resolver against live DNS. A change is
tested by saving values live DNS never returns (names under .invalid,
192.0.2.1) in the state a check starts from, or by marking a real
nameserver failed. The timeout test queries 192.0.2.1, where nothing
answers. The live-DNS retry and concurrency limit moved from the
resolver tests to internal/livedns, so both packages share them.
NewFromLoggerWithClient had no other use and is gone. TESTING.md and
the DNSClient comment now state the README's rule.

Model: opus-5-5

TESTING.md

@@ -2,8 +2,12 @@
 
 ## DNS Resolution Tests
 
-All resolver tests **MUST** use live queries against real DNS servers.
-No mocking of the DNS client layer is permitted.
+DNS is never mocked in this project, not in tests and not anywhere
+else; see the README section "No DNS mocking. Ever." Every test that
+looks something up in DNS **MUST** query live DNS servers, never a
+stand-in. Logic that works on record data, such as comparing or
+formatting records, may be tested on that data directly with no
+lookup.
 
 ### Rationale
 
@@ -21,14 +25,18 @@ real servers ensures the resolver works correctly in production.
 - Query timeout is calibrated to 3× maximum antipodal RTT (~300ms)
   plus processing margin
 - Root server fan-out is limited to reduce parallel query load
+- Live lookups that expect an answer go through `internal/livedns`,
+  which limits how many run at once in a test binary and retries a
+  lookup that got none
 - Flaky failures from transient network issues are acceptable and
   should be investigated as potential resolver bugs, not papered over
   with mocks or skip flags
 
 ### What NOT to do
 
-- **Do not mock `DNSClient`** for resolver tests (the mock constructor
-  exists for unit-testing other packages that consume the resolver)
+- **Do not mock, fake or stub DNS** anywhere: no stand-in `DNSClient`,
+  no stand-in for the watcher's `DNSResolver`, no fake DNS server, no
+  canned responses
 - **Do not add `-short` flags** to skip slow tests
 - **Do not increase `-timeout`** to hide hanging queries
 - **Do not remove `-count=1` from `script/test`** — Go's test cache