Move rclrs_test into rclrs

[maspe36]

As @esteve explained in our March 11th monthly meeting, rclrs_tests does not need to be a separate package. It was originally created as a separate package before there was a pattern of vendoring message packages.

Now that we are able to do this, there is no merit to rclrs_tests being its own package, the contents of this package are intended to be unit tests for rclrs.