tinycrypt: Solve style issues

This commit solves some indentation issues detected in the following
test-cases/files:

- test_cbc/src/test_cbc_mode.c
- test_ctr/src/test_ctr_mode.c
- test_prng/src/test_hmac_prng.c

Change-Id: I0f184ae4b510e4552a061523ea3e1216ae1f6f2a
Signed-off-by: Flavio Santes <flavio.santes@intel.com>