Added fuzzing spa packet generation for invalid encodings

Added fuzzing spa packet generation for invalid encodings

This commit adds the ability to generate SPA packets that are valid except for
the last encoding step before encryption.  This is independent of supplying
invalid data for SPA packet fields.  To invoke the test suite in this mode,
do something like:

 # ./test-fwknop.pl --enable-perl-module-pkt-gen  --fuzzing-test-tag "encoded_colon1_missing"  --fuzzing-class encoding

This assumes that lib/fko_encode.c has been patched to subvert the encoding
step itself before encryption.  In this case, the first colon after the random
value is removed.