Use --load-extension to set up for contrib/tcn's isolation tests.

Use --load-extension to set up for contrib/tcn's isolation tests.

Oversight in commit 418ec3207: it's better to do it like this,
else you have to drop and recreate the extension for each
permutation.  tcn.spec only has one permutation at present,
so this doesn't speed it up any, but it's still a bad example.