Make SRFI 64 test-group set up a runner if needed

[a2379]

Currently, this code will error:

(test-group "my-tests"
  (test-assert #t))

test-group should install a test runner if one does not exist.

[shirok]

Thanks. This prompted me to reread the SRFI document, and I realize test-begin also need to install a runner if there's none, which will be uninstalled by matching test-end. I'll take this PR as is and will revise it from there.