Correctly use WP_TESTS_DOMAIN in unit tests

[johnbillion]

When the WP_TESTS_DOMAIN constant is changed, some unit tests fail because they hard code example.org.

[johnbillion]

In 34519:

Use WP_TESTS_DOMAIN where it should be used in tests in place of hard-coded uses of example.org.

Also corrects a test value in the data provider for Tests_Sanitize_Option::test_sanitize_option() which was only consequentially passing.

Fixes #34000

[johnbillion]

In 35346:

Correct some more tests which were using example.org instead of WP_TESTS_DOMAIN.

See #33641, #34000

[johnbillion]

In 35347:

Correctly use WP_TESTS_EMAIL in email tests.

See #761, #34000

[johnbillion]

In 36717:

Correct some more tests which were using example.org instead of WP_TESTS_DOMAIN.

See #34000