test: move 'http' tests into subfolder

This PR moves all of the parallel/test-http-* files into parallel/http/test-*. If this change goes well, I look to implement it throughout the entire parallel folder (one subsystem at a time)

IMO, the DX will be much easier with this change, as searching through tests will become simpler, as users can simply visit the test/parallel/<subsystem> directory.

From a maintenance point of view, CODEOWNERS and label rules should be simpler, as a subdirectory now exists for these subsystems.

The http subsystem is largest subsystem (in number of test files), so I decided to handle it first.