Tests: add the missing usings (UniRx, System.Linq)

Fixes the compile errors of the previous commit; that commit does not build.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/Aetheria.Shared.Tests/ThrustAllocationTests.cs

@@ -6,6 +6,7 @@ using System;
 using System.Collections.Generic;
 using System.Linq;
 using CultMath;
+using UniRx;
 using Xunit;
 using static CultMath.math;
 using float2 = CultMath.float2;

tests/Aetheria.Shared.Tests/ThrustAllocatorTests.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using CultMath;
 using Xunit;