tests: constrain http pytest to tests/http directory

tests: constrain http pytest to tests/http directory

Running the http pytest had to be done from tests directory or above,
because the repeat argument fixture was defined in tests/conftest.py.
However, the repeat argument is not needed because its functionality
can be provided by pytest-repeat as documented in the test's
README.md. So, removed the pytest_addoption function for the repeat
argument and the pytest_report_header function is moved to
tests/http/conftest.py.

TODO: Remove repeat argument from all tests. As a stopgap, a
one-element list is defined for it for now.

Closes #14611