Add "mendertesting" tests to mender-convert, so we can enforce copyrights and changelog policy

The "mendertesting" tests are the tests that enforce the changelog tags, and ensure that license and copyright years are correct. The latter two won't be checked very thoroughly for non-Go code, but anyway.

Acceptance criteria:

• Run mendertesting tests in mender-convert, either in Jenkins or Travis
  • At least the check_commits.sh and check_license.sh must be run

This will enable us to run the changelog generator on mender-convert.