Create helper function for parsing runner configuration environment variables

[desrosj]

This introduces the setup_runner_env_vars() function, which processes all runner-related environment variables that are used in multiple scripts (prepare.php, test.php, etc.).

There are also some new default values being added for when certain environment variables are missing entirely. These new default values were initially added in #212, but they are being split out for separate consideration. This PR expands on the commit that introduced defaults a bit by also refining the expected/actual data types.

Other notes:

• WPT_DEBUG has changed from expecting a falsey value or verbose to expecting a falsey or truthy value. This should preserve the current behavior while making it more closely match Core, and be easier to understand as an on/off switch.
• WPT_CERTIFICATE_VALIDATION will now default to true when not set at all. This guards against a misconfiguration accidentally leaving the script potentially vulnerable.
• The processing of WPT_SSH_PRIVATE_KEY_BASE64 was left out of the function to avoid unintentionally exposing the value in the test, report, or cleanup scripts.

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Co-authored-by: desrosj <desrosj@git.wordpress.org>
Co-authored-by: kittenkamala <amykamala@git.wordpress.org>
Co-authored-by: Crixu <crixu@git.wordpress.org>

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