pg_regress supports a --multibyte option, which can also be invoked
using the MULTIBYTE make variable when you run make check. I think this
is fairly unknown, but we are now documenting it as part of how to run
the collation regression tests. I think it would also be useful in the
PL/Python regression tests for some of the unicode test variants.
So before this spreads too widely, should we rename it to --encoding and
ENCODING to be consistent with the naming everywhere else? Does anyone
use the old interface? Do we need to keep it?