Previously, an attribute disjunction erroneously either had no values for any of the
attributes, or it required a value for all attributes. A required value of null in an
attribute disjunction now specifies that the disjunction imposes no value on the attribute.

Put unit test helper functions in internal package to make them available to unit tests of all packages

Instead, a client state stored at testdata/teststorage is now used.

When a problem occurs during parsing or verifying a scheme manager or any of its underlying files, this is now stored in the scheme manager struct itself instead. Invalid managers are kept in Configuration.SchemeManagers.

Unit tests tended to sometimes fail when state from previously
(panic()'d) was still present in test storage, and when the test
storage directory did not exist. This should fix both.

This prevents the 'use of internal package not allowed' go compile error

This feature can no longer work because the scheme managers in the
irma_configuration of the old Android app are not signed, so they
would be rejected anyway.

The struct name now matches exactly (up to idiomatic spelling differences)
with the name of the folder that it parses and updates.