Support POST requests in runTest method

[max-ostapenko]

I'm using this package to build a CI for custom metrics code([related issue]).(HTTPArchive/custom-metrics#6)
But I get the 414 URI Too Long error though.

After debugging it with a dryRun - it looks this package doesn't support POST requests.

I add 14Kb custom metric code to test options and GET request to API obviously fails.
Would be great to implement support for POST requests in runTest method.

P.S. WPT API supports it.

[max-ostapenko]

Fixed in the fork HTTPArchive/WebPageTest.api-nodejs