PHPUnit 6 deprecated the `setExpectedException()` method in favor of the `expectException()`, `expectExceptionMessage()`, and `expectExceptionCode()` methods.
`WP_UnitTestCase_Base::setExpectedException()` backfilled the old method. As the PHPUnit Polyfills have a polyfill for the ''new'' method, this backfill can now be simplified.
This backfill ''should'' be removed in a future iteration, but is, for now, left in place so as not to break backward compatibility for plugin/theme test suites which extend the WP native test suite for their integration tests.
Follow-up to [48996], [48997], [51559-51561].
Props jrf.
See #46149.
git-svn-id: https://develop.svn.wordpress.org/trunk@51562 602fd350-edb4-49c9-b593-d223f7449a82