Move actual tests out of tracetools_test to new test_tracetools package

Part of #100 (closed)

This splits tracetools_test into two packages:

• tracetools_test: tracing-related test utilities (changed from a CMake package with C++ & Python code to a pure Python package)
• test_tracetools: tests for tracetools (CMake package with C++ & Python code)

I also updated the README and the CI config.

When doing the next release, we can make bloom ignore test_tracetools so that it's not released. See how it's done for launch_ros: https://github.com/ros2-gbp/launch_ros-release

Signed-off-by: Christophe Bedard bedard.christophe@gmail.com