tests: transformed tests/sha256 into a unittest

Partial fix for issue #1131.

tests/sha256 -> unittest/tests-crypto/tests-crypto-sha256.c
Whenever I compare strings in C I prefer to use ```strncmp```. Maybe thats just nit-picking but I think it
makes sense (especially in the context of a test). The ```TEST_ASSERT_EQUAL_STRING``` macro is essentially a ```strcmp```.