test: fix wait_until_ready in cluster:start()

The option was handled incorrectly in some cases.

Before this commit the behavior was the following.

| Code                                      | Waits? | Is it correct? |
| ----------------------------------------- | ------ | -------------- |
| cluster:start()                           | Yes    | Yes            |
| cluster:start({wait_until_ready = true})  | Yes    | Yes            |
| cluster:start({wait_until_ready = false}) | No     | Yes            |
| cluster:start({})                         | No     | !! No !!       |

Now it is the following.

| Code                                      | Waits? | Is it correct? |
| ----------------------------------------- | ------ | -------------- |
| cluster:start()                           | Yes    | Yes            |
| cluster:start({wait_until_ready = true})  | Yes    | Yes            |
| cluster:start({wait_until_ready = false}) | No     | Yes            |
| cluster:start({})                         | Yes    | !! Yes !!      |

There are no tests affected by this problem, but it worth to fix it just
in case.

NO_DOC=testing helper change
NO_CHANGELOG=see NO_DOC
NO_TEST=no code is added to tarantool itself