Add tests for custom functions

[capital-G]

Some things that should be tested

• try to compile script after loading, so the user gets immediate feedback about script errors (and not only when they create a DynGen instance). If compilation fails we bail out instead of trying to update potentially lots of existing DynGen instances with an erroneous script (which would lead to lots of duplicate error messages). [i.e. the old existing script gets continued instead of replacing it with a faulty one]
• bufRead and bufWrite should not map out-of-bound channel to channel 0; instead bufRead returns 0.0 and bufWrite simply does nothing.
• in and out should not map out-of-bound channel to channel 0; instead in(chan) returns 0.0 and assigning to out(chan) does nothing.

Originally posted by @capital-G in #81 (comment)

[Spacechild1]

If compilation fails we bail out instead of trying to update potentially lots of existing DynGen instances with an erroneous script (which would lead to lots of duplicate error messages).

How should we test that? We cannot really detect from the outside if a DynGen instance has attempted to compile an erroneous script, except for the console output.

[i.e. the old existing script gets continued instead of replacing it with a faulty one]

It would do that anyway.

[capital-G]

It would do that anyway.

Oh right - well, then simply test this behavior that replacing the script of a running DynGen unit with a faulty one does not affect its output ;) Seems like a good test case.