Add main configuration file for tests

[mrodm]

What does this PR do?

Adds a new configuration file to define global settings for tests: _dev/test/config.yml

Why is it important?

This configuration file is going to be used to define if tests of specific runners could be run in parallel or they should be run sequentially.

Example of the contents of this configuration file:

system:
  parallel: true
policy:
  parallel: false
  skip:
    reason: flaky tests
    link: https://github.com/elastic/package-spec/issues/1

Checklist

• [ ] I have added test packages to test/packages that prove my change is effective.
• [ ] I have added an entry in spec/changelog.yml.

Related issues

• Part of https://github.com/elastic/elastic-package/issues/787

[elasticmachine]

:green_heart: Build Succeeded

• Buildkite Build
• Commit: b939ffdff0d36bd60f0635ec2ae149ff6f1035cb

History

• :green_heart: Build #694 succeeded 8d065e83d2178ef76a9f6d94d1703d4d9e75541b
• :green_heart: Build #688 succeeded 1a3f98697f6e1aa529e331fdd12f45f95e101a1c
• :green_heart: Build #687 succeeded 2d76f9224b0b5ac1df568aee2ff08cb3ab88a5a3
• :green_heart: Build #684 succeeded ad8e94641a88d136bcb566255ad83843fc2b8817

cc @mrodm