test: Add a DST and tests reproducing found failures (#110)

Co-authored-by: Marcelo Soares <marcelo.me.soares@gmail.com>

packages/serverpod_offline_sync/test/hlc/hlc_fixtures.dart

@@ -0,0 +1,46 @@
+/// Constant fixtures for the `Hlc` tests.
+///
+/// Nothing here reads the wall clock or generates a random value, because both
+/// make an HLC test depend on when it runs.
+///
+/// `Hlc.increment` and `Hlc.merge` compare against `clock.now()` and reject a
+/// timestamp that drifts more than a minute from it, so a fixture built from
+/// `DateTime.now()` only passes while the run is close to the moment the
+/// fixture was created. On a runner that re-executes tests in the same process
+/// that stops being true, since a top-level `final` is initialized once per
+/// isolate and can outlive the values derived from it.
+///
+/// Node ids are fixed for the same reason: `Hlc.compareTo` breaks ties on the
+/// node uuid, so a random id makes the expected ordering vary between runs.
+library;
+
+import 'package:clock/clock.dart';
+import 'package:uuid/uuid.dart';
+
+/// The instant the HLC tests treat as the current wall time.
+final hlcTime = DateTime.parse('2001-09-09T01:46:40.000Z');
+
+/// A node id that sorts before [hlcSecondNodeId].
+final hlcNodeId = UuidValue.withValidation(
+  '11111111-1111-4111-8111-111111111111',
+);
+
+/// A node id that sorts after [hlcNodeId].
+final hlcSecondNodeId = UuidValue.withValidation(
+  '22222222-2222-4222-8222-222222222222',
+);
+
+/// Runs [body] with the wall clock pinned to [wallTime].
+T atWallTime<T>(DateTime wallTime, T Function() body) =>
+    withClock(Clock.fixed(wallTime), body);
+
+/// Offsets for building fixtures relative to [hlcTime].
+extension HlcFixtureTime on DateTime {
+  /// This instant one millisecond later.
+  DateTime advance([Duration duration = const Duration(milliseconds: 1)]) =>
+      add(duration);
+
+  /// This instant one millisecond earlier.
+  DateTime retreat([Duration duration = const Duration(milliseconds: 1)]) =>
+      subtract(duration);
+}