add tests

[maphew]

leoAst.py with repo history has been imported, now do the same for unit tests (and then make them work of course)

[maphew]

I skimmed over the Leo tests pertaining to AST but have no idea how to extract them out to use independently.

To import files from Leo-editor repo with their development history see ./tools/git-copy.sh script. It's how I processed leoAst.py (though I used the script as a guide and typed the commands).

No hurry to get started, e.g. it's probably wise to wait to do this until after the project & module name (#1) has settled.