test: correct entry point to pytest

With Pytest 6.0.2 'make tests' fails:

sandbox: Traceback (most recent call last):
  File "./test/py/test.py", line 20, in <module>
    sys.exit(load_entry_point('pytest', 'console_scripts', 'pytest')(args))
TypeError: console_main() takes 0 positional arguments but 1 was given

The definition of console_scripts has changed as follows:

Pytest 4.6.1:

[options.entry_points]
console_scripts =
        pytest=pytest:main
        py.test=pytest:main
        
Pytest 6.0.2:
        
[options.entry_points]
console_scripts =
    pytest=pytest:console_main
    py.test=pytest:console_main

The new function console_main() has a comment:
"This function is not meant for programmable use; use `main()`"

Hence let's call pytest.main() directly.
Move args processing into the __main__ paragraph.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Tested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Tested-by: Tom Rini <trini@konsulko.com>