fix yamllint reported errors under tests/
    
    With the fix some warnings are still left...
    
        # yamllint -d "{extends: default, rules: {line-length: {max: 120}, indentation: disable}}"  tests/
        tests/prepare.yml
        2:1       warning  missing document start "---"  (document-start)
    
        tests/avocado.yml
        2:1       warning  missing document start "---"  (document-start)
        13:20     warning  truthy value should be one of [false, true]  (truthy)
    
        tests/rhts.yml
        1:1       warning  missing document start "---"  (document-start)
        28:20     warning  truthy value should be one of [false, true]  (truthy)
    
        tests/inventory.yml
        1:1       warning  missing document start "---"  (document-start)
    
        tests/source.yml
        2:1       warning  missing document start "---"  (document-start)
    
        tests/basic.yml
        1:1       warning  missing document start "---"  (document-start)
        56:20     warning  truthy value should be one of [false, true]  (truthy)
        90:20     warning  truthy value should be one of [false, true]  (truthy)
    
        tests/beakerlib.yml
        1:1       warning  missing document start "---"  (document-start)
        30:20     warning  truthy value should be one of [false, true]  (truthy)
        63:20     warning  truthy value should be one of [false, true]  (truthy)
    
        tests/shared-tasks/verify_failed_test.yml
        2:1       warning  missing document start "---"  (document-start)
    
        tests/shared-tasks/req-pkg.yml
        1:1       warning  missing document start "---"  (document-start)
    
        tests/shared-tasks/artifacts_test_env.yml
        2:1       warning  missing document start "---"  (document-start)
    
        tests/shared-tasks/verify_error_test.yml
        2:1       warning  missing document start "---"  (document-start)
    
        tests/shared-tasks/artifacts_test_runner.yml
        2:1       warning  missing document start "---"  (document-start)
    
        tests/shared-tasks/artifacts.yml
        2:1       warning  missing document start "---"  (document-start)