tests: fix the HTTP/2 tests

The HTTP/2 tests brought with commit bf05606e were using the internal
name 'http2' for the HTTP/2 server, while in fact that name was already
used for the second instance of the HTTP server. This made tests using
the second instance (like test 2050) fail after a HTTP/2 test had run.

The server is now known as HTTP/2 internally and within the <server>
section in test cases. 1700, 1701 and 1702 were updated accordingly.