Increase shared user fixtures following switch to bcrypt hashing.

[peterwilsoncc]

Modifies a bunch of test suite users to be created as shared fixtures.

As there are a lot of uniquely generated fixtures, I think this ticket would best be served by working on the issue piecemeal rather than fixing the issue all at once.

These initial changes seem to have cut a few minutes off the test run but there's more to be done.

Trac ticket: https://core.trac.wordpress.org/ticket/63026

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