Features: provide a "check against defaults" function

[aarongable]

The features package currently provides a .reset() method, which the tests use extensively in order to ensure that any feature flags which were set by individual tests get cleared before other tests run.

However, this method often gets called in shared setup code, which can accidentally clear feature flags which were purposefully set by an individual test before it gets to the actual test logic.

The features package should also provide a "check against the defaults" method which returns an error if any flags have been modified. Then that can be used in the shared setup code, to error out and enforce that every individual test case cleans up after itself.