Good suggestion, David.  Here is such sample test code.  It is adapted from the sample code for "ValueError: generator already executing" included in PEP 255:

def test_gen(call_gen_method):
    def gen():
        call_gen_method(me)
        yield 1
    me = gen()

    try:
        me.__next__()
    except Exception as e:
        print(repr(e))

test_gen(lambda g: g.__next__())
test_gen(lambda g: g.send(1))
test_gen(lambda g: g.throw(OSError))
test_gen(lambda g: g.close())

This outputs:

ValueError('generator already executing',)
ValueError('generator already executing',)
ValueError('generator already executing',)
ValueError('generator already executing',)