add `disableFallbacks()` to disable development defaults

relying on development defaults in production/integration environments
can be nasty. add a method to disable them, and document a common
pattern for doing so
Please put this test into a separate file since it makes the order in which tests in this file are executed matter. Tests that are executed after this test will have fallbacks disabled.

Alternative (better) solution: Provide a `getenv.enableFallbacks()` method that resets the behavior!