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

[mity]

Currently, there is a compile-time constant limiting length of helper messages output via TEST_MSG 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_MSG_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-msg=NUMBER.

[RosscoG]

Hi @mity. Thank you for acutest, I've used if for years as my goto for any C I write.

I'm working on this issue now as my intro to open source contributions. I understand it's a pretty simple feature, but I'm comprehensively documenting the process as practice and to add to the pull request. I'm new to working on open source projects, so any feedback will be appreciated.

I see the equivalent issue for TEST_DUMP_MAXSIZE, #34.
Once this one is done, would you like that to also be implemented for consistency, or leave it for another beginner?