Added System namespace to BankAccountTests.cs

[mavaddat]

The BankAccountTests.cs needs the System namespace for the ArgumentOutOfRangeException class that is used in Create and run new test methods section.

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[PRMerger9]

@mavaddat : Thanks for your contribution! The author, @gewarren, has been notified to review your proposed change.

[PRMerger20]

@jborsecnik : Thanks for your contribution! The author, @gewarren, has been notified to review your proposed change.

[mavaddat]

Looks good. I forgot the indefinite article ('a') prior to the namespace, so I was hoping someone would throw that in. Thanks!