tests/node_ops: avoid interference betwen failure injections and ops

[bashtanov]

They can already be separated with a lock, but it's not sufficient. We need certain delay between them.

Fixes #18272

Backports Required

• [ ] 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
• [ ] v24.1.x
• [ ] v23.3.x
• [ ] v23.2.x

Release Notes