trivial: not an empty list, a list of valid authentication methods
trivial: not an LOA, an unrecognized authentication method
trivial: JavaDoc test cases and name test methods
Use test method naming to express what is tested.
tests extraction works when multiple supported authentication methods
tests extraction returns null when no authn_method specified
enhance test to test that resulting service reports the expected required authentication method
tests that when service parameter missing extractor extracts null service
Added a "sample_two_factor" authentication, demonstrating support for multiple methods.

Refactored context files, separating settings per authn method as much as possible.
removes now-duplicate misplaced test
Removes a unit test that exercised the Extractor rather than the Service.  Case now tested by testMissingServiceParameterYieldsNullService .
removes duplicate misplaced tests
Removes tests of the Extractor rather than of the Service, which duplicate tests of the Extractor elsewhere.
removes test of the Extractor rather than of the Service
Extractor exercised elsewhere, including exercising this test case.
renames test case, adds JavaDoc for test case
Added a "sample_two_factor" authentication, demonstrating support for multiple methods.

Refactored context files, separating settings per authn method as much as possible.
Added the ability to report unrecognized authentication methods upon login.
refactor to move extraction into extractor
Refactors to move ServletRequest-touching functionality for extracting information about a MfaService into the Extractor and out of the Service implementation.
fixes JavaDoc to reflect createCredentials() no longer returns null
stop asserting null where exception thrown in lieu of null
Noted where argument extractor configured.
strong_two_factor state id now uses underscores rather than dashes
Document `sample_two_factor` authentication method example.