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.