Checking for "twice the available memory limit" fails when running on a system with a memory_limit of -1

Actually, that entire assertion is useless.
It's provides no additional coverage since
$this->assertFalse(drupal_check_memory_limit('30MB', '16MB'), 'drupal_check_memory_limit() returns FALSE with a 16MB upper limit on a 30MB requirement.'); is a couple lines later.

Duh, I deleted the variable. Also, I'm unsure that this isn't a bug now, since I reread the test and there is:

// The function should always return true if the memory limit is set to -1.
$this->assertTrue(drupal_check_memory_limit($twice_avail_memory, -1), 'drupal_check_memory_limit() returns TRUE when a limit of -1 (none) is supplied');

Duh. The assertion I'm removing is stating that when the memory limit is twice as big, it will always fail. So that's just not true.

Just reupping the last patch, but also the example of how this breaks.

The failing patch was intended to fail. So you can test the one named 'commit'.

Keep in mind that this only fixes the test failure, not an actual bug.