[idna] Enable code to fail on IDNA2008 Bidi Rule 6 [B6]
[idna] Enable code to fail on IDNA2008 Bidi Rule 6 [B6]

* The code was disabled to allow tests pass. Now that `IdnaTest.txt` is
fixed for this failure, we can re-enable the code.

cGuille/rust-url,servo/rust-url,cGuille/rust-url,servo/rust-url