tests: net: http_server: Add HPACK testsuite

Add tests suite covering HTTP2 HPACK header compression and Huffman
encoding.

Note - many HPACK encoding/decoding cases in tests look similar, but
there are subtle differences in HPACK header used. For decoding, we
should be able to decode all of them. For encoding, we only test what
we support (no dynamic indexing).

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>