SRFI 64, 78: unit tests + Chicken/Chibi test

[lassik]

I found two testing-related SRFIs:

• SRFI 64: A Scheme API for test suites (2006)
  https://srfi.schemers.org/srfi-64/srfi-64.html
• SRFI 78: Lightweight testing
  https://srfi.schemers.org/srfi-78/srfi-78.html

Additionally, Chicken and Chibi both implement the following popular testing API, known as the Chicken test egg as well as (chibi test). Both APIs should be the same:

• http://wiki.call-cc.org/eggref/5/test
• http://snow-fort.org/s/gmail.com/alexshinn/chibi/test/0.7.3/index.html

We should probably support all three of the above APIs in addition to Gambit's native one.

[lassik]

@feeley I'll start porting these now.