Use a disposable private key in the 4337 tests

[leonardotc]

Context / issue

Currently, private keys for 4337 tests are used as CI secrets, most likely, to prevent random people to alter the state of chain in a way that break our tests hindering our operations. That introduces two issues:

• We either have toi share a (couple) of private key secrets internally everytime some change to the implemtantion breaks the tests or we recreate the same conditions for the associated eoas/multisig which takes some effort and suffering.
• externals will never, in their lives, be able to execute (and get runtime information from) those tests.

Proposed solution

• Add fixtures for private keys
• Mock the transport to avoid any consequences that could affect the result of those tests

Expected outcome

• No data should be read from process.env in the tests of the 4337 module.
• No secrets data should be contained on .github/workflows/sdk-test.yml
• The tests for the 4337 module should pass locally and on the CI.