fix: update @backstage/backend-test-utils to v0.5.1 to work with backstage v1.30.x

[jpd4emis]

Description

Bump @backstage/backend-test-utils to work with backstage v1.30.x as it requires @backstage/backend-app-api@npm:0.9.3

Issue number: PagerDuty/backstage-plugins#21

Type of change

• New feature (non-breaking change which adds functionality)
• Fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have performed a self-review of this change
• Changes have been tested
• Changes are documented
• Changes generate no new warnings
• PR title follows conventional commit semantics

If this is a breaking change 👇

• I have documented the migration process
• I have implemented necessary warnings (if it can live side by side)

Acknowledgement

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

Disclaimer: We value your time and bandwidth. As such, any pull requests created on non-triaged issues might not be successful.