Devops: Add tox environment that runs tests with sqlite backend

[agoscinski]

The problem with the current tox test environments is that they do require psql database which require a service running in the background. With the merge of #6425, we can use the sqlite backend to run tests more encapsulated with less dependencies on global configurations.

I still need to figure out how to set up a ssh config in a virtual-environment-friendly way. We could just run the .github/workflow/setup_ssh.sh file, but then it messes too much with existing ssh configuration. One could add an include in ${HOME}/.ssh/config to include another config file that is stored locally in the tox environment.

Include ${AIIDA_DEV_TOX_ENVIRONMENT}/custom_ssh_config

This just adds one line to the user config, that only works inside the tox environment. So does not have any effect outside of the environment to the functionality of ssh.