Runner cleanup - continuation (#209)

* Agent/AgentCredential -> Runner/RunnerCredential
* Test trait rename: Agent -> Runner
* Enable remaining RunnerL0 tests
* Remove job message PII variable masking code
* Remove unused Agent.ToolsDirectory variable
* Misc test Agent -> Runner renaming
* Some more misc cleaning

src/Test/L0/ConstantGenerationL0.cs

@@ -9,7 +9,7 @@ namespace GitHub.Runner.Common.Tests
     {
         [Fact]
         [Trait("Level", "L0")]
-        [Trait("Category", "Agent")]
+        [Trait("Category", "Runner")]
         public void BuildConstantGenerateSucceed()
         {
             List<string> validPackageNames = new List<string>()