Integration test for HTTP

[bbockelm]

For S3, we have true integration tests: we start a S3 server, an XRootD server, and pull data from S3 via XRootD.

This is not true for HTTP. For HTTP, we only start a single server (equivalent to the S3 server in the above case) and, instead of pulling data through an intermediate XRootD, we invoke C++ code directly against it.

Let's improve the HTTP test by adding a second XRootD server, allowing an end-to-end test.

Adding this to @alexandertuna's queue so he has a chance to see some of the CMake test fixture code.