Add optional test filter and improve error handling

[fadookie]

This is a fixed-up version of https://github.com/peartherapeutics/bitrise-aws-device-farm-runner/pull/40 which generated invalid json

While testing I discovered some serious issues with error handling so I have tried to fix them.

• Remove faulty settings save/restore code which didn't restore errexit setting in get_test_package_arn
• Seperate local variable definitions from assignment so return codes from subshells is not swallowed
• Set errexit and nounset for subshells