tests-httpd: basic infra to run curl against an apache httpd plus nghttpx for h3

tests-httpd: basic infra to run curl against an apache httpd plus nghttpx for h3

- adding '--with-test-httpd=<path>' to configure non-standard apache2
  install
- python env and base classes for running httpd
- basic tests for connectivity with h1/h2/h3
- adding test cases for truncated responses in http versions.
- adding goaway test for HTTP/3.
- adding "stuttering" tests with parallel downloads in chunks with
  varying delays between chunks.

- adding a curltest module to the httpd server, adding GOAWAY test.
    - mod_curltest now installs 2 handlers
      - 'echo': writing as response body what came as request body
      - 'tweak': with query parameters to tweak response behaviour
- marked known fails as skip for now

Closes #10175