Add tests for ASTQ#func and ASTQ#adapter methods

[cancerberoSgx]

Would be great to have a working test demonstrating ASTQ#adapter and ASTQ@func methods (particularly the later has no example in readme). I think at some point you wanted to add a test (

astq/tst/astq.js

Line 53 in 9764beb

astq.func("add", function (a, b) { return a + b })

) but that line can be removed and tests will still pass).

I will try to add tests for both APIs (ASTQ#func and ASTQ#adapter methods), For the later I will need to define a custom AST (probably similar to snippet's #9 (comment)). Would you accept a PR with these ? Thanks