Runtime-host meta tag never tested

[schlessera]

The runtime-host meta tag does not seem to be tested at all, as all the tests use a relative ampUrlPrefix and thus throw an exception in the related code.

https://github.com/ampproject/amp-toolbox/blob/3ec7bd40f34adbc8be66f951edfa6d91d5794c01/packages/optimizer/lib/transformers/RewriteAmpUrls.js#L116-L120

I assume this is either an oversight or a bug in the tests.