Add README for tests

tests/README.md

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+# esp-open-rtos tests
+
+Testing is based on [Unity](https://github.com/ThrowTheSwitch/Unity) 
+C testing framework. 
+
+## Features
+
+* Single device test case.
+* Dual devices test cases. Run test case on two ESP8266 modules simultaneously.
+* Run only specified test cases.
+* List available test cases on a device.
+
+## Usage
+
+There's a test runner script `test_runner.py` written in Python3 that runs
+test cases on ESP8266 connected to a host.
+
+### Options
+
+`--type` or `-t` - Type of test case to run. Can be 'solo' or 'dual'.
+If not specified 'solo' test will be run.
+
+`--aport` or `-a` - Serial port for device A. 
+If not specifid device `/dev/ttyUSB0` is used.
+
+`--bport` or `-b` - Serial port for device B. 
+If not specifid device `/dev/ttyUSB1` is used.
+
+`--no-flash` or `-n` - Do not flash the test firmware before running tests.
+
+`--list` or `-l` - Display list of the available test cases on the device.
+
+### Example
+
+Build test firmware, flash it using serial device `/dev/tty.wchusbserial1410`
+and run only *solo* test cases:
+`./test_runner.py -a /dev/tty.wchusbserial1410`
+
+Build test firmware. Flash both devices as `-t dual` is specifid. And run both
+*solo* and *dual* test cases:
+`./test_runner.py -a /dev/tty.wchusbserial1410 -b /dev/tty.wchusbserial1420 -t dual`
+
+Do not flash the firmware, only display available test cases on the device:
+`./test_runner.py -a /dev/tty.wchusbserial1410 -n -l`
+
+Do not flash the firmware and run only 2 and 4 test cases:
+`./test_runner.py -a /dev/tty.wchusbserial1410 -n 2 4`
+
+## References
+
+[Unity](https://github.com/ThrowTheSwitch/Unity) - Simple Unit Testing for C