Add tags option

[nprizal]

bktec is adding support for tagging test results with key=value metadata (buildkite/test-engine-client#530), and this adds a tags array option to the plugin so users can pass tags through to their uploaded results.

plugins:
  - tests#v1.0.0:
      test-runner: rspec
      tags:
        - "language.version=3.3"
        - "os=linux"

Buildkite exposes array plugin properties as indexed env vars (TAGS_0, TAGS_1, ...) but a single string value comes without an index. The environment hook handles both cases, joining multiple tags with commas to form the BUILDKITE_TEST_ENGINE_TAGS value that bktec expects.

Verification

Tested it using local build of bktec

  - name: "Jest"
    commands:
      - my/local/bktec run
    parallelism: 2
    plugins:
      - tests#40cb5e48a8e6b23b4a47d048fad1139a66cc79fb:
          test-runner: jest
          result-path: tmp/jest-result.json
          tags:
            - "my.tag=my.value"
            - "my.other.tag=my.other.value"