Get the tests for the 4.0 branch passing

[johnbillion]

tl;dr: Most of #30284 should be backported to the 4.0 branch (and earlier).

The test suite did not pass in versions prior to 4.1. Failing tests were committed to the test suite but skipped with the now-defunct knownWPBug notation.

Work on recent tickets such as #35105, #39486, and #40086 means the 4.1 to 4.6 branches are passing once again, but 4.0 (and earlier) basically needs most of the changes in #30284 applying to it.

Having a passing test suite in the 4.0 branch means we can be more confident in patch releases.

Related: #40100, #40106.

[johnbillion]

I've started this already with [40445], [40446], [40447], [40448], and [40449].

[ocean90]

Related: ticket:35105:43

[johnbillion]

In 40450:

Remove failing unit tests from 'canonical' group.

Each removed test has been turned into a patch and posted to the open ticket
that it belongs to.

See #30284.

Merges [30282] to the 4.0 branch.

See #40463

[johnbillion]

In 40451:

Remove skipped tests for unimplemented JS plural functions.

See #22229.

Merges [30517] to the 4.0 branch.

See #40463

[johnbillion]

In 40452:

Remove failing test in the hooks group.

See #30284.

Merges [30519] to the 4.0 branch.

See #40463

[johnbillion]

In 40453:

Remove failing tests related to square brackets being stripped in URL sanitizers.

See #30284.

Merges [30514] to the 4.0 branch.

See #40463

[johnbillion]

In 40454:

Remove failing is_textdomain_loaded() test.

See #30284.

Merges [30521] to the 4.0 branch.

See #40463

[johnbillion]

In 40455:

Remove failing tests related to WP_User::__unset().

See #30284.

Merges [30513] to the 4.0 branch.

See #40463

[johnbillion]

In 40456:

Better rewrite rule flushing in test_permalink_without_title().

This ensures that the test is not polluted by permalink structure changes in
earlier tests.

Merges [30529] to the 4.0 branch.

See #40463

[johnbillion]

In 40457:

Build/Test tools: Ruthlessly remove failing tests from the 4.0 branch.

The corresponding issues that these failing tests relate to were fixed during 4.1 development. The tests will never pass on the 4.0 brach.

See #40463

[johnbillion]

I forgot to include a reference to this ticket in [41306], which fixed this.