Only build system.buffers.tests for now, to avoid disk full noise in CI

Author: Jo Shields

src/libraries/tests.proj

@@ -389,7 +389,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <ProjectReference Include="$(MSBuildThisFileDirectory)*\tests\**\*.Tests.csproj"
+    <ProjectReference Include="$(MSBuildThisFileDirectory)*\tests\**\System.Buffers.Tests.csproj"
                       Exclude="@(ProjectExclusions)"
                       Condition="'$(TestAssemblies)' == 'true'" />
     <ProjectReference Include="$(MSBuildThisFileDirectory)pkg\test\testPackages.proj"