[tests] add an inspector for travis certification tests (#3700)

This commit adds a debugger to inspect nodes status.

tests/scripts/thread-cert/README.md

@@ -0,0 +1,50 @@
+OpenThread Certification Tests
+==============================
+
+Inspector
+--------
+
+Inspect nodes status by the following modification:
+
+1. Insert the inspector to where you want to inspect.
+
+```python
+import debug
+debug.Inspector(self).inspect()
+```
+
+2. Run the test and it will stop at the line above and prompt `#`.
+
+```sh
+./script/test clean build cert tests/scripts/thread-cert/Cert_5_1_01_RouterAttach.py
+```
+
+3. Inspect
+
+```sh
+#
+# 1
+> state
+leader
+> exit
+# 2
+> panid
+face
+> exit
+# exit
+```
+
+### CLI reference
+
+#### `#` mode
+This is selection mode. You may select the node to inspect here.
+
+- `list`      - list available nodes.
+- `exit`      - end inspecting, continue running test case.
+- \<number\>   - select the node with id \<number\>. This will result in entering `>`  mode.
+
+#### `>` mode
+This is node mode. You may run OpenThread CLI here.
+
+* `exit`      - go back to `#` mode.
+