Introduced Awaitility to handle async test code

[mackdk]

What does this PR change?

This PR introduces Awaitility, a new test-only dependency to handle and simplify async code in tests. This change is meant to ease async code testing and reduce flakyness.

GUI diff

No difference.

• DONE

Documentation

• No documentation needed: only internal and user invisible changes

• DONE

Test coverage

• Unit tests were updated

• DONE

Changelogs

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Re-run a test

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• Re-run test "changelog_test"
• Re-run test "backend_unittests_pgsql"
• Re-run test "java_pgsql_tests"
• Re-run test "schema_migration_test_pgsql"
• Re-run test "susemanager_unittests"
• Re-run test "javascript_lint"
• Re-run test "spacecmd_unittests"

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