Runtime-host meta tag never tested

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.

Start at packages/optimizer/lib/transformers/RewriteAmpUrls.js around lines 116-120 and inspect the existing tests that use a relative ampUrlPrefix. Reproduce the exception and determine how the runtime-host meta tag path is expected to behave; done means the relevant behavior is covered by a passing test.

Written by the indexing model from the issue text.