[Conformance] Expand valid email tests to cover semi-valid email address values

[rodaine]

If implementations of the isEmail custom CEL expression use an address parsing library, the input might contain extraneous whitespace that the parser automatically cleans (e.g., " foo@bar.com ".isEmail() might evaluate to true). This goes against the spirit of the field.string.email constraint. We should add similar conformance tests that cover these cases (a'la the fix for this bug in protovalidate-go).