exp/locale/collate: let regtest generate its own collation table.

exp/locale/collate: let regtest generate its own collation table.
The main table will need to get a slightly different collation table as the one
used by regtest, as the regtest is based on the standard UCA DUCET, while
the locale-specific tables are all based on a CLDR root table.
This change allows changing the table without affecting the regression test.

R=r
CC=golang-dev
https://golang.org/cl/6453089