Update tests.
Reason: it turns out that the native output generated by
pretty-simple isn't always readable by the native reader.
According to https://github.com/cdepillabout/pretty-simple/issues/99
it is not a design goal of the library that the rendered values
be readable using 'read'. This makes it unsuitable for our
purposes.
pretty-show is a bit slower and it uses 4-space indents
(non-configurable), but it doesn't have this serious drawback.
