Remove .sln files from test assets, use only .slnx

- Deleted MultiTestProjectSolutionWithPlatforms.sln
- Deleted MultiTestProjectSolutionWithSharedProject.sln
- Updated platform configuration test to use .slnx file
- Updated slnx Build="false" attribute syntax (removed Project parameter)

Note: Platform configuration test currently failing as slnx Build="false" attribute
may not be fully supported yet in the SDK's solution parsing logic.

Co-authored-by: Youssef1313 <[email protected]>

Author: Copilot

test/TestAssets/TestProjects/MultiTestProjectSolutionWithPlatforms/MultiTestProjectSolutionWithPlatforms.sln

@@ -7,6 +7,6 @@
   </Configurations>
   <Project Path="TestProject/TestProject.csproj" />
   <Project Path="OtherTestProject/OtherTestProject.csproj">
-    <Platform Solution="*|x86" Project="Any CPU" Build="false" />
+    <Platform Solution="*|x86" Build="false" />
   </Project>
 </Solution>

test/TestAssets/TestProjects/MultiTestProjectSolutionWithPlatforms/MultiTestProjectSolutionWithPlatforms.slnx

@@ -596,7 +596,7 @@ public void RunWithSolutionAndPlatformConfiguration_ShouldRespectPlatform(string
         {
             TestAsset testInstance = _testAssetsManager.CopyTestAsset("MultiTestProjectSolutionWithPlatforms", Guid.NewGuid().ToString()).WithSource();
 
-            string testSolutionPath = "MultiTestProjectSolutionWithPlatforms.sln";
+            string testSolutionPath = "MultiTestProjectSolutionWithPlatforms.slnx";
 
             // Test with x86 platform - OtherTestProject should NOT be included (no Build.0 for x86)
             CommandResult resultX86 = new DotnetTestCommand(Log, disableNewOutput: false)