REST API: Use `REST_TESTS_IMPOSSIBLY_HIGH_NUMBER` in Users Controller test

[danielbachhuber]

100 isn't arbitrarily high enough but REST_TESTS_IMPOSSIBLY_HIGH_NUMBER is.

[danielbachhuber]

In 43767:

REST API: Use a truly impossibly high number in User Controller tests.

The 100 number could be valid in certain test run configurations. The REST_TESTS_IMPOSSIBLY_HIGH_NUMBER constant is impossibly high for this very reason.

Fixes #45128.

[danielbachhuber]

Reopening for merge to trunk.

[desrosj]

In 44126:

REST API: Introduce Autosaves controller and endpoint.

• Adds WP_REST_Autosaves_Controller which extends WP_REST_Revisions_Controller.
• Autosaves endpoint is registered for all post types except attachment because even post types without revisions enabled are expected to autosave.
• Because setting the DOING_AUTOSAVE constant pollutes the test suite, autosaves tests are run last. We may want to improve upon this later.

Also, use a truly impossibly high number in User Controller tests. The number 100, (or 7777 in trunk), could be valid in certain test run configurations. The REST_TESTS_IMPOSSIBLY_HIGH_NUMBER constant is impossibly high for this very reason.

Finally, Skip Autosaves controller test for multisite. There's a PHP 5.2 edge case where paths calculated differently, possibly caused by differing version of PHPUnit.

Props adamsilverstein, aduth, azaozz, danielbachhuber, rmccue, danielbachhuber.

Merges [43767], [43768], [43769] to trunk.

See #45132, #45131.
Fixes #45128, #43316.