test_retry_non_txn slow with serializable&non-pushable

Adding to @bdarnell's comment in `test_retry_non_txn`:

```
# The write/write can't-push test cases take 15 seconds each.
# TODO: why? because the go version uses Store.SetRangeRetryOptions?
#(Methods.Put, data_pb2.SNAPSHOT, False, 1),
#(Methods.Put, data_pb2.SERIALIZABLE, False, 1),
# Read/write conflicts.
```

I have found that the test cases present still incur a 15sec wait for me, unless I comment the last one:

```
test_cases = [
# Write/write conflicts.
(Methods.Put, data_pb2.SNAPSHOT, True, 2),
(Methods.Put, data_pb2.SERIALIZABLE, True, 2),
# The write/write can't-push test cases take 15 seconds each.
# TODO: why? because the go version uses Store.SetRangeRetryOptions?
#(Methods.Put, data_pb2.SNAPSHOT, False, 1),
#(Methods.Put, data_pb2.SERIALIZABLE, False, 1),
# Read/write conflicts.
(Methods.Get, data_pb2.SNAPSHOT, True, 1),
(Methods.Get, data_pb2.SERIALIZABLE, True, 2),
(Methods.Get, data_pb2.SNAPSHOT, False, 1),
#(Methods.Get, data_pb2.SERIALIZABLE, False, 1),
]
```

This test appears to be reliably slow (~19sec for all tests) with the last case, and fast (~4sec for all tests) without it.