Adds explicit --name argument to test-ref

In addition, `./mach test` will now take advantage of this when
inferring test suites from test files.
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)