Increase test coverage for aescbc and cbc

[braydonf]

Functions and branches in aescbc.js and cdb.js are not covered in tests:

File	Statements	Branches	Functions	Lines
aescbc.js	75.76% (25 / 33)	50% (1 / 2)	40% (2 / 5)	75.76% (25 / 33)
cbc.js	92.94% (79 / 85)	33.33% (2 / 6)	91.67% (11 / 12)	92.94% (79 / 85)