slapos/test/recipe/test_slaposconfiguration: fix tests for Python 2.7

Python 2 jsonschema uses repr() for unicode values in error messages, producing u'...' prefixes (e.g. u'number' instead of 'number'), and Python 2 dicts have no guaranteed key ordering.

Add assertErrorEqual() helper that normalises u'...' prefixes on Python 2 before comparing, and use it for all error-message assertions.

For test_report_error_shared_multiple_invalid, check the multi-key dict error by parts on Python 2 since repr key order is not guaranteed.

For test_report_error_main_no_json_path_notation, adjust the expected message to omit the unevaluatedProperties line on Python 2: jsonschema 3 (used on Python 2) does not support that keyword, so only the type error is produced.