Add docs/TESTING.md from Claro v1.17.26 zip

docs/TESTING.md

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+# Testing Claro
+
+Run the built-in regression suite:
+
+```bash
+./claro test
+```
+
+Expected result:
+
+```text
+PASS: 0 failure(s)
+```
+
+Check a lesson:
+
+```bash
+./claro check lessons/01_hello.claro
+```
+
+Validate an RC3 package after building:
+
+```bash
+python tools/validate_rc3.py
+```
+
+On Windows, after `build.bat` or `build.ps1`:
+
+```powershell
+python toolsalidate_rc3.py
+```