Add in unit test for dev/mailing#56 and dev/mailing#57 and also fix i…

[seamuslee001]

…ssue where by mailing_name has been namespaced also in 5.20

Overview

This adds a unit test to verify the fix for dev/mailing#56 and dev/mailing#57. It also handles another situation of the same problem which is that mailing_name has been namespaced in 5.20 08d08c0 and when running the unit test locally without the changes in CRM/Mailing/BAO/Mailing a new notice was thrown

Before

No unit test and e-notice thrown when running mailing report about mailing_name

After

unit test and no e-notice

ping @eileenmcnaughton @monishdeb @demeritcowboy

[civibot]

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