internal(Thresholds): polish UI

[cristianoventura]

Description

This PR addresses some UI inconsistencies and bugs on the Thresholds feature.

• Tooltip in the URLs Select component
• Rename "Stop" column to "Stop test"
• Fix isRunning state in Validator Dialog when a threshold crosses
• Remember last selected tab in Test Options

How to Test

• Enable the Thresholds feature
• Check the Thresholds tab in Test Options

Checklist

• I have performed a self-review of my code.
• I have added tests for my changes.
• I have run linter locally (npm run lint) and all checks pass.
• I have run tests locally (npm test) and all tests pass.
• I have commented on my code, particularly in hard-to-understand areas.

Screenshots (if appropriate):

Related PR(s)/Issue(s)