Feature: Update `onlyOwnersGuard` to use `O(1)` lookup, fix an incorrect test

[mmv08]

This PR:

• Update the example onlyOwnersGuard to use O(1) owner lookup instead of O(n)
• The test for the guard's "happy" path was incorrect because it signed a transaction with user2, which wasn't an owner and the safe instance was still connected to the owner's signer. The transaction failed with GS026, Safe's built-in error for an incorrect owner. The guard actually didn't stop the transaction

[github-actions[bot]]

Pull Request Test Coverage Report for Build 6836209786

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 94.567%

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Totals
Change from base Build 6720462527:	0.3%
Covered Lines:	397
Relevant Lines:	405

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💛 - Coveralls