Support to take the user-defined test report template

[LinuxSuRen]

Currently, all the test reports are embed in the code lines. But it's quite normal that uesrs want a new test report style. A simple solution is that allowing pass a user-defined template.

A suggestion is adding a flag to the following command:

atest run --report-template (my-template.md | my-template.html | https://foo/my-template.html)

References

• api-testing/pkg/runner/writer_markdown.go

  Line 33 in 02d7e23

  var markdownReport string

• api-testing/pkg/runner/writer_html.go

  Line 33 in 02d7e23

  var htmlReport string