Changes

unittests/test_files: Pass to-be-encoded length to encoder functions

Some of the encoders can only generate valid output if they are told
the expected output size.  This is due to variable-length fields that
depend on the size of the total record (or file).  Let's always pass
the expected length to the encoder methods.

Change-Id: I88f957e49b0c88a121a266d3582e79822fa0e214