The extra HTTP header fields are compiled from
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers,
https://en.wikipedia.org/wiki/List_of_HTTP_header_fields, and
https://www.owasp.org/index.php/List_of_useful_HTTP_headers.

Since v0.6.2-7-g1d138acc ("Unify DATA and other frames in
nghttp2_outbound_item and save malloc()"), the macros are unused and the
builds fails on -Werror=unused-macros.

`go test` requires both config.go and the test files in the same
directory. For out-of-tree builds, config.go is normally not placed next
to the source files, so copy the tests to the build directory as a
workaround.

Fixes multiple errors while making docs:

    Could not import extension sphinxcontrib.rubydomain (exception: No module named 'sphinxcontrib')

and

    ../../doc/sources/index.rst:15: WARNING: toctree contains reference to nonexisting document 'nghttp.1'
    ../../doc/sources/index.rst:15: WARNING: toctree contains reference to nonexisting document 'nghttpd.1'
    ../../doc/sources/index.rst:15: WARNING: toctree contains reference to nonexisting document 'nghttpx.1'
    ../../doc/sources/index.rst:15: WARNING: toctree contains reference to nonexisting document 'h2load.1'
    ../../doc/sources/index.rst:15: WARNING: toctree contains reference to nonexisting document 'programmers-guide'