Ensure "Machine Testable" guidelines are actually deterministically machine-testable

[jyasskin]

For example:

1. https://w3c.github.io/sustainableweb-wsg/#success-criterion-barriers-and-access-machine-testable links to https://w3c.github.io/sustainableweb-wsg/star.html#UX02-3. That technique's procedure starts with "Identify visitors or individuals with disabilities who can help you become more accessible.", which is not something a computer can do.
2. https://w3c.github.io/sustainableweb-wsg/#success-criterion-semantic-code-machine-testable says that "Content must be accurately marked up according to the relevant standard(s).", but its test, https://w3c.github.io/sustainableweb-wsg/star.html#WD08-1, is only about fixing validation errors. That's not a test that would fail if you used <span> where <a> makes more sense.
3. https://w3c.github.io/sustainableweb-wsg/#success-criterion-asynchronous-code-machine-testable links to https://w3c.github.io/sustainableweb-wsg/star.html#WD09-1 where the "unless necessary" isn't something a machine could know about.
4. https://w3c.github.io/sustainableweb-wsg/#success-criterion-script-security-machine-testable links to https://w3c.github.io/sustainableweb-wsg/star.html#WD16-1 which says to "Check the product or service for vulnerabilities within your code, ensuring they are resolved." Anyone who can implement that in an automated tester would be the hero of our industry.
5. https://w3c.github.io/sustainableweb-wsg/#success-criterion-refresh-frequency-machine-testable links to https://w3c.github.io/sustainableweb-wsg/star.html#HIS07-1, which is titled "Ensure That Stored Data Has a Defined and Reasonable Expiry Date". "Reasonable" is not something a machine can check. The "procedure" for that technique actually has nothing to do with its title, but is instead an implementation of expiration dates.