Disable TFM-parallel test runs; mark GC test as explicit

Set TestTfmsInParallel to false to serialize test runs per project, reducing resource usage in .NET 9+ SDKs. Marked a GC memory release test as explicit so it only runs when explicitly requested, preventing it from running by default.

Author: kzdev-net

Source/Tst/Directory.Build.targets

@@ -5,5 +5,7 @@
     -->
     <PropertyGroup Condition="'$(IsTestProject)' == 'true'">
         <RunSettingsFilePath>$(MSBuildThisFileDirectory)KZDev.PerfUtils.tests.runsettings</RunSettingsFilePath>
+        <!-- .NET 9+ SDK runs each TFM in parallel by default; serialize per project for fewer hosts. -->
+        <TestTfmsInParallel>false</TestTfmsInParallel>
     </PropertyGroup>
 </Project>

Source/Tst/KZDev.PerfUtils.Memory.Heap.Sequential.UnitTests/UsingMemoryStreamSlimReleaseBuffers.cs

@@ -41,7 +41,8 @@ public UsingMemoryStreamSlimReleaseBuffers (ITestOutputHelper xUnitTestOutputHel
     /// to using unmanaged memory, so this test is contained in the heap specific test
     /// project.
     /// </remarks>
-    [Fact]
+    [Fact(Explicit = true)]
+    [Trait(TestConstants.TestTrait.TestMode, TestConstants.TestMode.Explicit)]
     public void UsingMemoryStreamSlimReleaseBuffers_CopyFullToStream_SettingReleaseMemoryProperlyReleasesMemory ()
     {
         int[] testDataSizes = GenerateTestDataSizes(1000, 0xF_FFFF).ToArray();