Add timer to tests

[willGraham01]

Adds a timer to the system tests

[Copilot]

Pull Request Overview

This PR adds execution timing functionality to the system tests by tracking how long each test command takes to run and displaying the elapsed time to the user.

• Replaces print statements with sys.stdout.write for output formatting
• Implements timer logic using time.time() to measure test execution duration
• Displays elapsed time in seconds after each test completes

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[samcunliffe]

No output in logs 😞

[willGraham01]

Output is now visible in the logs: https://github.com/UCL/TDMS/actions/runs/17826245607/job/50680792730#step:9:17

[samcunliffe]

You've still got too many sig. fig.s to my taste. And elapsed not ellapsed (annoyingly elipse ellipse is the other way around).

But now working 👍