XMLHttpRequest test fixes

[jungkees]

• abort-event-loadend.htm: changed to call abort() in onloadstart rather than in onreadystatechange; otherwise the test ends up with throwing InvalidStateError in send().
• getresponseheader-error-state.htm: corrected the URL composition part; without "//", location.protocol ("http:") tends to be interpreted as [protoco] + [host] + [path] by user agent, which leads the result to 404 error instead of CORS failure.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/333

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