Add `ForceSetShardMaster` option to `AddTablet*` testutil functions
Add `ForceSetShardMaster` option to `AddTablet*` testutil functions

This allows conveniently setting up a shard with multiple tablets
claiming to be MASTER, with the shard be correctly configured to have
_a_ serving master. Previously, those were mutually-exclusive setups
with `AddTablets`.

Signed-off-by: Andrew Mason <8a240ff75aa1467ba5eba4db0c8200b61262547e@slack-corp.com>

vitessio/vitess,vitessio/vitess,mahak/vitess,mahak/vitess,mahak/vitess,mahak/vitess,vitessio/vitess,mahak/vitess,mahak/vitess,vitessio/vitess,vitessio/vitess,tinyspeck/vitess,tinyspeck/vitess,tinyspeck/vitess,vitessio/vitess,mahak/vitess,tinyspeck/vitess,tinyspeck/vitess,tinyspeck/vitess,vitessio/vitess,tinyspeck/vitess,vitessio/vitess,mahak/vitess