Clarify description in prolog_test.go

I didn't understand my old comment so others probably didn't either.
Use library(tap) format for pure Prolog tests

Each one of Golog's pure Prolog tests is now also a valid library(tap)
test.  This lets me run the entire test suite with SWI-Prolog to make
sure the tests are right.  Once they're behaving correctly, I can make
Golog pass those tests too.

This is another step towards making these pure Prolog tests into a
generic Prolog acceptance suite.  A Prolog implementation must support
library(tap) and then it gets all these tests for free.
Support compound terms with quoted names

A compound term like 'hello world'(message) is valid but we had been
parsing it incorrectly.  This is a common pattern in library(tap)
tests, which is how it came up.
Implement ground/1
Implement phrase/2
Implement memberchk/2

It's a system predicate in SWI-Prolog but very easy to implement in
Prolog.  I chose the latter route.  It'd be faster as a built in, but
I'm not convinced it's performance critical.
go fmt