OPFS VFSes: remove the on-open() pragma calls, as those (A) already reflected the...

OPFS VFSes: remove the on-open() pragma calls, as those (A) already reflected the build-time default settings and (B) they made it illegal to run locking_mode=exclusive, which is a requirement for WAL mode without shared memory. Modify part of the test suite to demonstrate that the SAHPool VFS can run in WAL mode so long as locking_mode=exclusive is used.

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