Add ability to dump test env vars to file

[sfod]

Add possibility to dump testing environment variables to a specified file. The dump file is specified via an environment variable AWS_SETUP_CRT_TEST_ENVIRONMENT_DUMP_FILE.

This is useful when you want to setup your local environment for running tests. Execute the following commands from the repo root to set env vars:

export AWS_SETUP_CRT_TEST_ENVIRONMENT_DUMP_FILE=~/work/setup_test_env.inc
builder run SetupCrossCICrtEnvironment -p tests
source ~/work/setup_test_env.inc

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.