Fix session + Serializable tests

These were broken when the deprecation warning was added, as the
extra output prevented the session from being started.

Add some missing EXTENSIONS sections to misc tests

Partially deprecate Serializable

If Serializable is implemented, require that __serialize() and
__unserialize() are implemented as well, else issue a deprecation
warning.

Al

Partially deprecate Serializable

If Serializable is implemented, require that __serialize() and
__unserialize() are implemented as well, else issue a deprecation
warning.

Also deprecate use of PDO::FETCH_SERIALIZE.

RFC: https://wiki.php.net/rfc/phase_out_serializable

Closes GH-6494.

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Add test for bug #79031

Fixed by preceding revert.