software/slapos-sr-testing: generate an environment script
Using an script to set the environment is easier to use, but the main reason is that if we want to be able to use shared part from outer slapos (like when using in theia or slaprunner), the working directory for test must be contained in the instance directory of the outer slapos, so that when we run slapos node prune in the outer slapos this command can see that shared parts are used.