Make `a[i] = x` specs test Array#[]= return value

Previously, many of the Array#[]= specs were only testing the return
value of the expression `a[*args] = rhs`. However, that expression
is equivalent to `lambda { a.send(:[]=, *args, rhs); return rhs }`,
which means that the return value of Array#[]= wasn't actually being
tested. (I.e., `obj[*args] = rhs` returns `rhs` *regardless* of the
return value of `obj.[]=`.)

Specs testing assignment syntax sugar appear to belong in
language/variables_spec.rb

Relevant corrections to that file will be committed separately.