Adds explicit --name argument to test-ref

In addition, `./mach test` will now take advantage of this when
inferring test suites from test files.
style::properties::longhand shouldn’t be public.
Remove dead code
Fix the panic on runnig with --devtools due to a change in how type_id gets resolved since the last rustc upgrade.
Don't abort the sniffer task whenever an error occurs. Also, switch to blocking recv_opt vs. non-blocking try_recv.
auto merge of #4148 : jdm/servo/sniffix, r=glennw

...locking recv_opt vs. non-blocking try_recv.
auto merge of #4147 : jdm/servo/devtoolspanic, r=Manishearth

...ets resolved since the last rustc upgrade.
auto merge of #4141 : SimonSapin/servo/style-cleanup, r=jdm
auto merge of #4093 : mttr/servo/mach_reftest_by_file, r=jdm

`./mach test-ref` no longer eats the first "-" prefixed argument for test name filtering, instead using an explicit "--name" argument. This should avoid any potential confusion when passing down arguments to servo.

Also allows for ./tests/ref to be included (and trimmed away) for tab completion convenience (`./mach test` has also been updated to take advantage of this when inferring tests from file paths).

(fixes #4091)
Fixes #4123
auto merge of #4149 : aakashjain/servo/MovingElementGetters, r=Manishearth

Moved all getters from Element to ElementHelpers. Existing ElementHelpers getters `get_namespace` and `get_local_name` were replaced by `namespace` and `local_name`. Callers were updated accordingly. Also the getters are no longer inlined.
3 of the getters needed to be added to RawLayoutElementHelpers as well, to accomodate existing calls directly from Element objects.