For beginners: Change TEST_DUMP_MAXSIZE into a run-time limit.

[mity]

Currently, there is a compile-time constant limiting length of helper messages output via TEST_DUMP macro. It would be good to change it so that people do not have to re-compile of they need to see longer message.

Because Acutest should not output too long things by default, it still should have a limit, but it should be changed into run-time one. I'd suggest that TEST_DUMP_MAXSIZE will be kept but will only determine the default length and user could specify another one by a new command line option, say --max-dump=NUMBER.

(Similar to #33.)