Add further tests

[mreithub]

Test the extension (using pg_regress)

A few things to test:

• test if recall_disable() restores the previous state of the DB
• check if ALTER TABLE statements work as expected (+ check if it breaks inserts into the log table as schema changes might create a disparity between data and log table)
• a stress test with a reasonable amount of data
• see if table/column name escaping works properly
• check if updating the primary key works as expected (mark the old log entry deleted, create a new one)
• ...

Done:

• the simplest case: recall_enable() and a few INSERT/UPDATE/DELETEs (with a few checks if the log table contains the right data).
• test recall_clean()
• unconditional UPDATE (i.e. one without a WHERE clause; effectively updating all rows at once)
• intentionally trigger the now() issue

[mreithub]

Also test if enable/disable works with tables that have foreign keys from/to other tables.