syntax: remove dead @mut Visitor impl (only used in tests).

Also, move some code only uses in the tests into the test module, and
replace some needless @mut ~[] with plain ~[].
Handle another possible error in a unix pipe test

This cropped up on the bsd bot, and if it's an error that gets thrown then it's
fine to just whitelist another type of error in the test.
auto merge of #10133 : alexcrichton/rust/another-error, r=thestinger

This cropped up on the bsd bot, and if it's an error that gets thrown then it's
fine to just whitelist another type of error in the test.
auto merge of #10117 : huonw/rust/dead-visits, r=sanxiyn

Used nowhere, and these are likely incorrect anyway: self needs to be
dereferenced once more otherwise the method calls will be reusing the
current impl... bam! Infinite recursion.
Field identifiers now include specific spans (Closes #7718).