[FLINK-27756] Refactor Async Sink send gauge test

[vahmed-hamdy]

What is the purpose of the change

Refactor checkLoggedSendTimesAreWithinBounds for Async Sink, the test introduced static upper bound that was violated due to mailbox executer delays causing multiple CI fails.
The new test format introduces artificial delay prior to send calculation and uses test measured times as upper bounds.

Brief change log

(for example:)

• refactor checkLoggedSendTimesAreWithinBounds test case in AsyncSinkWiterTest

Verifying this change

This change is already covered by existing tests, such as (please describe tests).

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? NA

[flinkbot]

CI report:

• c01ef94 Azure: SUCCESS

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