Generate tests from list of contexts

[dag]

Similar to #105 but generative. Each context represents a call of each test function in the collection rather than nested contexts.

Thanks ronny.

[dag]

The #105 equivalent would thus be something like,

db = Tests(foreach=[memorydb, filedb])

@db.test
def dbtest(conn):
    assert some_condition(using=conn)

The benefit becomes more obvious when the list of contexts is dynamic or large.