`storage`: fix flakey `TestTombstonesCompletelyEmptyLog`

[WillemKauf]

Adjust the sleep time as well as the tombstone_retention_ms value used to ensure self-compaction and tombstone removal occurs as expected.

Running with --gtest_repeat --gtest_break_on_failure locally shows no flakes post fix.

Backports Required

• [X] none - not a bug fix
• [ ] none - this is a backport
• [ ] none - issue does not exist in previous branches
• [ ] none - papercut/not impactful enough to backport
• [ ] v24.2.x
• [ ] v24.1.x
• [ ] v23.3.x

Release Notes

• none