Reorganize tests

- Pester source code moved to `test/Pester`, deleted `ext-src`.
- Pester tests (.ps1 files) moved to `test/powershell`
- xUnit tests (.cs files) moved to `test/csharp`
- Third-party script test moved to `test/shebang`

test/csharp/project.json

@@ -0,0 +1,17 @@
+{
+    "version": "1.0.0-*",
+    "description": "PowerShell On Linux xUnit Tests",
+    "authors": [ "andschwa" ],
+
+    "dependencies": {
+	"NETStandard.Library": "1.0.0-rc2-23712",
+        "Microsoft.PowerShell.Linux.Host": {
+	    "type": "build",
+	    "version": "1.0.0-*"
+	},
+	"xunit": "2.1.0-*",
+        "xunit.runner.dnx": "2.1.0-*"
+    },
+
+    "frameworks": { "dnxcore50": { } }
+}