Simplify MailAddressDisplayNameTest

[ManickaP]

Follow up of #37874
Many of the tests in MailAddressDisplayNameTest.cs are duplicates of the same test, only with different inputs.
--> Change the tests from [Fact] to [Theory] and parametrize them with [MemberData].

Some more clean up:

• some asserts have swapped expected and actual arguments: https://github.com/dotnet/runtime/blob/master/src/libraries/System.Net.Mail/tests/Unit/MailAddressTests/MailAddressDisplayNameTest.cs#L44
• local variable with undesired underscore: https://github.com/dotnet/runtime/blob/master/src/libraries/System.Net.Mail/tests/Unit/MailAddressTests/MailAddressDisplayNameTest.cs#L63

Improvements to consider:

• reparse MailAddress from ToString result (depends on fixed issue 36752 mail address display name #37874)

cc: @jaymin93 @karelz @wfurt

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[jaymin93]

should i do pr for that since i added test i am aware of test case and usage i think i can take this one and make it as expected

[ManickaP]

I'd first focus on finishing #37874. After that, you can tackle this one if you wish.