NO-JIRA refactor sns publisher service test

What does this MR do?

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Merge request backend checklist

• Self review is complete by author
• Merge request title is correct and is clear. i.e. HPPC-0000 added bank details screen
• Relevant milestone has been set
• On merge commits are squashed (feature branches only)
• For merging to main squash commits is disabled
• Build is passing
• If sonar has run any new code smells or issues have been fixed
• If the MR starts with NO-JIRA there are no functional changes that would cause behaviour to be any different
• The playbook standards have been followed - https://nhsbsa.github.io/nhsbsa-digital-playbook/development
• Coding standards are https://bsa2468.atlassian.net/wiki/spaces/NCH/pages/2218197003/Code+Quality+Standard+1.1+DRAFT+-+CL and follows google styling https://google.github.io/styleguide/javaguide.html or https://google.github.io/styleguide/jsguide.html
• There are no additional changes other than the story to be covered
• The commit message is named correctly and is clear to understand i.e. HPPC-0000 added api endpoint should be HPPC-0000 added api endpoint to update bank details
• The code matches the ACs defined in the story
• There are no additional changes covered other than the ACs mentioned

Others

• The README has been updated and is in clear English without any grammatical errors is correct, no use of tabs