DAL: reduce flakiness of amplification test

What

Reduce flakiness of the observer triggers amplification (without lost shards) test.

Why

The test <Proto>: Testing DAL node (observer triggers amplification (without lost shards)) has become flaky, probably after !18139 (merged). Indeed, in this test, receiving all shards should happen before amplification starts. With minimal_block_delay = 1, amplification may start 1 second after the first shard is received, which results in flakiness. With minimal_block_delay = 3, it may start only after 4s (see !18139 (merged)), which should be enough time for the observer to receive all the shards.

How

Just use a higher value for minimal_block_delay.

Manually testing the MR

Tested it with --loop-count 50 -j 2. With minimal_block_delay = 2, I've got a failure at iteration 47, so I increased the value to 3 and got no failures.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR