Add missing test pair cleanup to SharedGasSpecificHeatsTest (#42763)

* add SharedGasSpecificHeatsTest testpair cleanup

and make it nonparallel because sometimes the basic tests blast through so fast that the pairs are reused before they're read (or something? I honestly don't know, but making it non-parallel fixed issues I was seeing)

* Apply suggestion from @Centronias

Content.IntegrationTests/Tests/Atmos/SharedGasSpecificHeatsTest.cs

@@ -13,7 +13,7 @@ namespace Content.IntegrationTests.Tests.Atmos;
 /// Tests for asserting that various gas specific heat operations agree with each other and do not deviate
 /// across client and server.
 /// </summary>
-[TestOf(nameof(SharedAtmosphereSystem))]
+[TestOf(nameof(SharedAtmosphereSystem)), FixtureLifeCycle(LifeCycle.InstancePerTestCase)]
 public sealed class SharedGasSpecificHeatsTest
 {
     private IConfigurationManager _sConfig;
@@ -46,6 +46,12 @@ public sealed class SharedGasSpecificHeatsTest
         _cAtmos = _cEntMan.System<AtmosphereSystem>();
     }
 
+    [TearDown]
+    public async Task TearDown()
+    {
+        await _pair.CleanReturnAsync();
+    }
+
     /// <summary>
     /// Asserts that the cached gas specific heat arrays agree with each other.
     /// </summary>