Generated Bash test programs have no way for user to set environment variables

If a test needs to, for example, use ssh to connect to another host, it may need to access the ssh agent, which it finds via an environment variable. However, the generated test program sets a minimal, artificial environment at start, which is good, but could do with a way for the user to pass in specific environment variable values.

• Add --env key=value support to the bash runner
• Add --env key=value support to the python runner
• Add SUBPLOT_ENV_key=value support to the rust codegen