Fix leap_15_5 common_channel in testsuite

[jordimassaguerpla]

What does this PR change?

I did the mistake of using opensuse_15.5 instead of opensuse_15_5 ( . instead of _ ).
This PR fixes it.

GUI diff

No difference.

• DONE

Documentation

• No documentation needed
• DONE

Test coverage

• No tests

• DONE

Links

• DONE

Changelogs

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