test-reports: adding the meta-ros-test-reports git repository as git submodule

In the future, all test reports are supposed to be tracked and archived in the
meta-ros-test-reports git repository. This git repository is linked into the
meta-ros with a git submodule in the directory test-reports.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@oss.bmw-carit.de>