Improve customError testing

[Amxx]

Currently, if the promise does not fail, expect.fail("Expected promise to throw but it didn't"); is triggered but instantly catched. Since this message doesn't follow the expected format:

VM Exception while processing transaction: reverted with custom error 'ErrorName(params)'

the following line fails

const [, error] = revert.message.match(/'(.*)'/);

with

TypeError: object null is not iterable (cannot read property Symbol(Symbol.iterator))

[changeset-bot]

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