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/powershell/ConvertTo-SecureString.Tests.ps1

@@ -0,0 +1,11 @@
+Describe "ConvertTo-SecureString" {
+
+    Context "Checking return types of ConvertTo-SecureString" {
+
+        It "Should return System.Security.SecureString after converting plaintext variable"{
+            $PesterTestConvert = (ConvertTo-SecureString "plaintextpester" -AsPlainText -force)
+ 	    ($PesterTestConvert).GetType() | Should Be securestring
+
+	}
+    }
+}