Block cipher fixups

Various block cipher testing fixups. Some of these changes reflect edge
cases I encountered when running local GSC builds.

Change:
* Extend ciphertext lengths.
* Add SCOPED_TRACE() within for loops.
* Use '\t' instead of 'a' for PKCS7 padding.

Test: CTS/VTS

Signed-off-by: Brian J Murray <brianjmurray@google.com>
Change-Id: I4555519787e0133367ad3f40609d43a7bc71c36e