[perl #43310] [TODO] config/auto/readline.pm: Write unit tests

[James Keenan via RT]

In r24672, considerable refactoring of guts of runstep() to make it more
testable.  This was done along same lines as in RT 43312
(config/auto/gmp.pm) and, in fact, some code in common was refactored
out into new Parrot module Parrot::Configure::Step::Methods.  The stub
test file t/configure/144-auto_readline-01.t was fleshed out and a
second test file added.