Misuse of SpiClient::select in some tests

[syvb]

The documentation for SpiClient::select/SpiClient::update says that select should be used for operations that only read, and update should be used for operations that write. Currently all of our tests use select for all queries, even those that write to the database. The queries that write should be changed to use update.

This isn't much of a problem currently since select and update have the same implementations, but in future versions of pgx using select to modify the database might not work.

[syvb]

We fixed this in d498dc6 by replacing all uses of SpiClient::select with SpiClient::update. (Ideally we would still use select for read-only queries, but right now we only use SPI in tests so it's not very important.)