Add log compare test for devicekey 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\devicekey.bin --compare-log tests\devicekey.log
+```
+
+
+More details about `htrun` are [here](https://github.com/ARMmbed/htrun#testing-mbed-os-examples).
+

tests/devicekey.log

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+--- Mbed OS DeviceKey example ---
+
+--- Using the following salt for key derivation: SALT1 ----- SALT1 ------ SALT1 ---
+--- First call to derive key, requesting derived key of 16 byte ---
+--- Derived key1 is:
+[0-9A-F]+
+
+--- Second call to derive key with the same salt. ---
+--- Derived key2 should be equal to key1 from the first call. key2 is:
+[0-9A-F]+
+--- Keys match ---
+
+--- Using the following salt for key derivation SALT2 ----- SALT2 ------ SALT2 ---
+--- Third call to derive key with the different salt should result with a new derived key1:
+[0-9A-F]+
+--- Keys not match ---
+
+--- 32 byte key derivation example. ---
+--- 32 byte derived key is:
+[0-9A-F]+
+
+--- Mbed OS DeviceKey example done. ---