fix paymaster unit test folder and mutation readme

[mooncitydev]

what was wrong

• the paymaster unit tests lived under test/unit/concrete/paymaster/signaturePaymster (typo) while package.json script test:enso_checkout:unit uses paymaster/signaturePaymaster. forge --match-path did not match any SignaturePaymaster tests, so that npm command skipped the entire paymaster suite without failing loudly.
• the mutation testing section referenced mutationTest.mjs, which does not exist in the repo; the real entry point is testGambitMutations.mjs.
• the optional automation link pointed at ./scripts/... instead of ./test/scripts/....

what this does

• rename the folder to signaturePaymaster so it matches package.json and the documented layout.
• align the readme with the actual script name and fix the relative link.

made by mooncitydev