test: serialize AcDream.Core.Net.Tests so real-time transport waits are not starved

PausedSelector_SeededDroppedServerReady_RecoversOnIdleSweep failed twice in CI
on the Linux runner, taking 37 s and 42 s, while passing 5/5 in ~350 ms in
isolation on that same machine. The test drives a virtual clock but asserts on
real-time 2 s SpinUntil windows, so full-assembly parallelism on a 6-core
container starves it.

Passing -- xUnit.MaxParallelThreads=2 through dotnet test did not take effect.
A xunit.runner.json is read by xUnit directly and is the convention already
used by AcDream.Core.Tests and AcDream.Launcher.Tests.

tests/AcDream.Core.Net.Tests/xunit.runner.json

@@ -0,0 +1,6 @@
+{
+  "$schema": "https://xunit.net/schema/current/xunit.runner.schema.json",
+  "parallelizeAssembly": false,
+  "parallelizeTestCollections": false,
+  "maxParallelThreads": 1
+}