Refactor email test with table-driven test and add more cases

[daisy1754]

Fixes Or Enhances

Added a few test cases around domains that contain number. One thing I realized was validator currently reject address like example.com1 which I believe is allowed per RFC but practically doesn't exist in IANA list. I wasn't sure if it's bug or feature, but at least having test case make it obvious for future reader.

Also refactored TestEmail to use Table Driven Test to avoid code duplication and make test result obvious. Before this, test failed with --- FAIL: TestEmail (0.00s), now it will be like

    --- PASS: TestEmail/Email:_"test_test"@email.com (0.00s)
    --- PASS: TestEmail/Email:_"@email.com (0.00s)
    --- FAIL: TestEmail/Email:_example@email.com1 (0.00s)

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• Tests exist or have been written that cover this particular change.

@go-playground/validator-maintainers

[coveralls]

coverage: 74.291%. remained the same
when pulling ea408f3 on daisy1754:kazuki/more_email_test
into a947377 on go-playground:master.