pgsql: 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.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a1b9b14cc80327854ff667973096a230434a2386

Modified Files
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contrib/tcn/Makefile       | 1 +
contrib/tcn/specs/tcn.spec | 1 -
2 files changed, 1 insertion(+), 1 deletion(-)