Add go_env make target

This target shows a list of environment variables to export in the shell
to run custom go commands and tests.
Split out common test utility functions

Also rename the Job test cases to avoid future name clashes and enable
test selection.
backend: Add Pool component
backend/job: More precise temp file prefix

As we are using temporary files elsewhere (e.g., for creating local
sockets), this patch make the naming of temporary input files clearer.