Add skip_until_version test helper

[sambostock]

This helper will skip the test ONLY if the gem version is below the specified version.

This ensures that changes meant for a particular version release are included, and not forgotten or otherwise missed. If the gem version is bumped to the specified version or more, the tests will not be skipped, forcing the behaviour to be implemented, or explicitly further deferred.

Specifically, we have a number of breaking changes queued up which we want to make sure to release together in v2.0.0, including:

• enforcing cursor serializability (see Reevaluate rollout of enforced cursor serializability #80)
• enforcing object wrapped by ThrottleEnumerator is itself an Enumerator (see Add test for future enforcement of Enumerator class in ThrottleEnumerator #149)
• optimizing ThrottleEnumerator to check if enumeration should be throttled before fetching next element (see 1.3.2: Defer throttle edge case optimization #148)

⚠️ Before Merging

• Rebase after Add test for future enforcement of Enumerator class in ThrottleEnumerator #149 is merged and update its tests to use this helper
• Rebase after 1.3.2: Defer throttle edge case optimization #148 is merged and update its tests to use this helper