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/shebang/README.txt

@@ -0,0 +1,8 @@
+Usage:
+
+PATH=$PATH:/absolute/path/to/powershell/app_base ./script.ps1
+
+Comment:
+
+The script assumes that /usr/bin/env can execute runps-file.sh.
+

test/shebang/script.ps1

@@ -0,0 +1,5 @@
+#!/usr/bin/env powershell
+
+get-process
+get-module
+