Expose an environmental variable so that SUT can change behavior

Problem/Motivation

Over in simpletest_example it would be great to be able to show the user a failing test in the UI, but still have the test pass under the testbot: #2565181: Simpletest Example test should support DrupalCI

At PNWDS a few years back @Mixologic and I discussed adding an environmental variable to mark this.

Upsides: The example would be cool.

Downsides: You start making promises to support all kinds of edge cases.

Proposed resolution

Way back in the day, we decided on:

if (getenv('DRUPALCI')) {
  etc.
}

This would mean adding an environmental variable that appears in the container for the Simpletest plugin.

Remaining tasks

User interface changes

API changes

Data model changes