build: add pytest targets

It enables running pytests in cmake jobs, regardless of underlying build
tool choice (= makes it work with ninja.)

Also:

- drop pytest logic launching `make` and exiting in case of failure.
  Maybe there is a better way and keep this functionality somehow, bind
  it to a command-line option? make it fail softly?

- GHA/linux: invoke pytest via the build, not directly.

- autotools: add missing dummy runtests targets when cross-compiling.

Closes #15034

tests/http/conftest.py

@@ -73,8 +73,6 @@ def env(pytestconfig) -> Env:
         pytest.skip(env.incomplete_reason())
 
     env.setup()
-    if not env.make_clients():
-        pytest.exit(1)
     return env
 
 @pytest.fixture(scope="package", autouse=True)