add new variation helper

add new variation helper

I'm using a lot of @testing.combinations with either
a boolean True/False, or a series of string names, each indicating
some case to switch on.  I want a descriptive name in the test
run (not True/False) and I don't want to compare strings.

So make a new helper around @combinations that provides an
object interface that has booleans inside of it, prints nicely
in the test output, raises an error if you name the case
incorrectly.

Before:

test/orm/declarative/test_typed_mapping.py::MappedColumnTest::test_separate_name[False-False-both] PASSED
test/orm/declarative/test_typed_mapping.py::MappedColumnTest::test_separate_name[False-False-key] PASSED
test/orm/declarative/test_typed_mapping.py::MappedColumnTest::test_separate_name[False-False-name] PASSED
test/orm/declarative/test_typed_mapping.py::MappedColumnTest::test_separate_name[False-True-both] PASSED
test/orm/declarative/test_typed_mapping.py::MappedColumnTest::test_separate_name[False-True-key] PASSED
test/orm/declarative/test_typed_mapping.py::MappedColumnTest::test_separate_name[False-True-name] PASSED
test/orm/declarative/test_typed_mapping.py::MappedColumnTest::test_separate_name[True-False-both] PASSED
test/orm/declarative/test_typed_mapping.py::MappedColumnTest::test_separate_name[True-False-key] PASSED
test/orm/declarative/test_typed_mapping.py::MappedColumnTest::test_separate_name[True-False-name] PASSED
test/orm/declarative/test_typed_mapping.py::MappedColumnTest::test_separate_name[True-True-both] PASSED
test/orm/declarative/test_typed_mapping.py::MappedColumnTest::test_separate_name[True-True-key] PASSED
test/orm/declarative/test_typed_mapping.py::MappedColumnTest::test_separate_name[True-True-name] PASSED

After:

test/orm/declarative/test_typed_mapping.py::MappedColumnTest::test_separate_name[not_use_add_property-deferred-both] PASSED
test/orm/declarative/test_typed_mapping.py::MappedColumnTest::test_separate_name[not_use_add_property-deferred-key] PASSED
test/orm/declarative/test_typed_mapping.py::MappedColumnTest::test_separate_name[not_use_add_property-deferred-name] PASSED
test/orm/declarative/test_typed_mapping.py::MappedColumnTest::test_separate_name[not_use_add_property-not_deferred-both] PASSED
test/orm/declarative/test_typed_mapping.py::MappedColumnTest::test_separate_name[not_use_add_property-not_deferred-key] PASSED
test/orm/declarative/test_typed_mapping.py::MappedColumnTest::test_separate_name[not_use_add_property-not_deferred-name] PASSED
test/orm/declarative/test_typed_mapping.py::MappedColumnTest::test_separate_name[use_add_property-deferred-both] PASSED
test/orm/declarative/test_typed_mapping.py::MappedColumnTest::test_separate_name[use_add_property-deferred-key] PASSED
test/orm/declarative/test_typed_mapping.py::MappedColumnTest::test_separate_name[use_add_property-deferred-name] PASSED
test/orm/declarative/test_typed_mapping.py::MappedColumnTest::test_separate_name[use_add_property-not_deferred-both] PASSED
test/orm/declarative/test_typed_mapping.py::MappedColumnTest::test_separate_name[use_add_property-not_deferred-key] PASSED
test/orm/declarative/test_typed_mapping.py::MappedColumnTest::test_separate_name[use_add_property-not_deferred-name] PASSED

Change-Id: Idde87632581ee69e0f47360966758583dfd8baab