Improve project structure

Author: ehonda

.github/instructions/tunit.instructions.md

@@ -1,5 +1,5 @@
 ---
-applyTo: '**/*.cs,**/*.csproj,**/*.slnx'
+applyTo: 'src/**,tests/**,**/Directory.*.props,Optional.slnx'
 ---
 # TUnit Instructions
 

Optional.slnx

@@ -2,8 +2,7 @@
   <Folder Name="/src/">
     <Project Path="src/Core/Core.csproj" />
   </Folder>
-  <Folder Name="/tests/" />
-  <Folder Name="/tests/Core.Tests/">
-    <Project Path="tests/Core.Tests/Tests.csproj" />
+  <Folder Name="/tests/">
+    <Project Path="tests/Core.Tests/Core.Tests.csproj" />
   </Folder>
 </Solution>