exp/locale/collate: add code to ignore tests with (unpaired) surrogates.

exp/locale/collate: add code to ignore tests with (unpaired) surrogates.
In the regtest data, surrogates are assigned primary weights based on
the surrogate code point value.  Go now converts surrogates to FFFD, however,
meaning that the primary weight is based on this code point instead.
This change drops tests with surrogates and lets the tests pass.

R=r
CC=golang-dev
https://golang.org/cl/6461100