Expose minimum test duration for WPT scripting.

[Nooshu]

#349 allowed 404 pages to be tested via the use of the 'min test duration' workaround, but this currently isn't possible via scripting alone.

Example usage:

log 1
time 2
navigate https://www.site.com/this-is-a-404-page

Also related: #1275

[pmeenan]

Decided to make the script command somewhat more descriptive than the api parameter. It's setMinimumStepSeconds:

setMinimumStepSeconds	20
navigate	https://www.gov.uk/this-is-a-404

[pmeenan]

Just pushed to the release branch and should roll out to the public instance over the next hour.