Testing for WP_DEBUG

[jarednova]

As noted in #2327, we should consider methods to test when WP_DEBUG is disabled in order to catch potential errors or issues involved in its setting

[acobster]

One way we could potentially do this is at the CI level, where WP_DEBUG is defined in a bootstrap file according to an environment variable. This would double the compute resources for every test run, though, so I'm not totally convinced it's worth it. The "blast radius" for WP_DEBUG being enabled vs. disabled is not even close to the size of the entire codebase, so a 2x increase in resources needed to test this indicates a design problem to me.

If we're really invested in testing these different code paths, we could abstract over WP_DEBUG with an internal filter, and hook into them from each test for the features we care about:

public function test_with_debug() {
  $stuff_is_normal = do_normal_stuff();
  $this->assertTrue( $stuff_is_normal );
}

public function test_with_debug_disabled() {
  $this->add_filter_temporarily( 'timber/internal/debug', '__return_false' );
  $stuff_is_normal = do_normal_stuff();
  $this->assertFalse( $stuff_is_normal );
}

[jarednova]

@acobster I think for now your suggestion on using the bootstrap file is a good answer for "consider ...". We could perhaps run this once a week or some other regular interval so it doesn't slow down normal test execution. A good thing to consider, but not high priority versus other stuff right now