Add command-line opts to enable/disable CI mode

[macdjord]

Currently, PyTest detects when it is running in a CI pipeline by checking the environment variables $CI and $BUILD_NUMBER, and this is the only way CI mode can be controlled.

There should be command-line options to enable/disable CI mode, which supersede auto-detection based on env variables.

This ticket is split off from #13766 so that that ticket can be used to cover improvements to the env-based autodetection, while this one deals with the proposal to add command-line overrides for it.

[RonnyPfannschmidt]

given

export CI=test && echo CI=$CI && env -u CI bash -c 'echo CI=$CI'   
CI=test
CI=

is there a actual need to add a additional cli arg? in particular as that would allow people to break what could be considered reasonable defaults by adding them into configfiles by accident (addopts=...)