http2-tests: test1700 is the first real HTTP/2 test

It requires that 'nghttpx' is in the PATH, and it will run the tests
using nghttpx as a front-end proxy in front of the standard HTTP/1 test
server. This uses HTTP/2 over plain TCP.

If you like me have nghttpx installed in a custom path, you can run test 1700
like this:

$ PATH=$PATH:$HOME/build-nghttp2/bin/ ./runtests.pl 1700