tests/http: more tests with specific clients

tests/http: more tests with specific clients

- Makefile support for building test specific clients in tests/http/clients
- auto-make of clients when invoking pytest
- added test_09_02 for server PUSH_PROMISEs using clients/h2-serverpush
- added test_02_21 for lib based downloads and pausing/unpausing transfers

curl url parser:
- added internal method `curl_url_set_authority()` for setting the
  authority part of a url (used for PUSH_PROMISE)

http2:
- made logging of PUSH_PROMISE handling nicer

Placing python test requirements in requirements.txt files
- separate files to base test suite and http tests since use
  and module lists differ
- using the files in the gh workflows

websocket test cases, fixes for we and bufq
- bufq: account for spare chunks in space calculation
- bufq: reset chunks that are skipped empty
- ws: correctly encode frames with 126 bytes payload
- ws: update frame meta information on first call of collect
  callback that fills user buffer
- test client ws-data: some test/reporting improvements

Closes #11006