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: #89

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)

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