add log compare test for blinky example

Author: Qinghao Shi

tests/README.md

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+# Testing examples
+
+Examples are tested using tool [htrun](https://github.com/ARMmbed/mbed-os-tools/tree/master/packages/mbed-host-tests) and templated print log. 
+
+To run the test, use following command after you build the example:
+```
+mbedhtrun -d D: -p COM4 -m K64F -f .\BUILD\K64F\GCC_ARM\blinky.bin --compare-log tests\blinky.log
+```
+
+
+More details about `htrun` are [here](https://github.com/ARMmbed/htrun#testing-mbed-os-examples).
+

tests/blinky.log

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+=============================== SYSTEM INFO  ================================
+Mbed OS Version:
+CPU ID: 0x[0-9a-fA-F]+
+Compiler ID: 1
+Compiler Version:
+================= CPU STATS =================
+Idle: \d+% Usage: \d+%
+================ HEAP STATS =================
+Current heap: \d+
+Max heap size: \d+
+================ THREAD STATS ===============
+ID: 0x[0-9a-fA-F]+
+Name: main
+State: \d+
+Priority: \d+
+Stack Size: \d+
+Stack Space: \d+
+ID: 0x[0-9a-fA-F]+
+Name: rtx_idle
+State: \d+
+Priority: \d+
+Stack Size: \d+
+Stack Space: \d+
+ID: 0x[0-9a-fA-F]+
+Name: rtx_timer
+State: \d+
+Priority: \d+
+Stack Size: \d+
+Stack Space: \d+