Stabilize shell serialization tests

[aibrahim-oai]

What changed

• The duration-recording fixture sleep was reduced from a large artificial delay to 0.2s, and the assertion floor was lowered to 0.1s.
• The shell tool fixtures now force login = false so they do not invoke login-shell startup paths.

Why this fixes the flake

• The old tests were paying for two kinds of noise that had nothing to do with the feature being validated: oversized sleep time and variable shell initialization cost.
• Login shells can pick up runner-specific startup files and incur inconsistent startup latency.
• The test only needs to prove that we record a nontrivial duration and preserve shell output. A shorter fixture delay plus a non-login shell keeps that coverage while removing runner-dependent wall-clock variance.

Scope

• Test-only change.