ES6 test functions in multi-line strings

This updates the build script to interpret ES6 code encoded in function
comment strings (see #190 discussion), which is then wrapped in
try-catch and eval() structures automatically.

Also, the runtime script has been updated to unwrap those same
structures for the tooltip preview, making the resulting display
cleaner.

Only some ES6 tests have been converted to make use of this format, for
the moment.