tinycrypt: Rename current tests to avoid confusions with new algorithms

Rename test_ctr to test_ctr_mode to avoid confusions with any CTR-based
algorithm. For example, currently TinyCrypt v0.2.0 supports the
CTR PRNG algorithm.

Rename test_prng to test_hmac_prng to avoid confusions with the test
case for the CTR PRNG algorithm or any test-case related to PRNGs.

Rename test_cbc to test_cbc_mode. TinyCrypt test case for CBC mode is
"test_cbc_mode.c", so it makes sense to keep the same name for the
directory.

Furthermore, README files are also updated to reflect the previous
described changes.

Change-Id: If0af4dcffe8917a7c29608231d3954b73dff5e10
Signed-off-by: Flavio Santes <flavio.santes@intel.com>