Python test fails when installing with DESTDIR

[cottsay]

Environment

• OS Version: Any
• Source or binary build? Source @ 2b66954

Description

• Expected behavior: Tests execute successfully
• Actual behavior: basic_TEST.py can't find gz.msgs10 to import

Steps to reproduce

1. mkdir -p build && cd build && cmake .. && make
2. make install DESTDIR=/tmp/foobar
3. ctest

Output

The following tests FAILED:
	 13 - basic_TEST.py (Failed)

---

The test sets PYTHONPATH based on where it anticipates the Python module will be installed to, however:

1. In general, a developer can typically invoke ctest prior to installing a project and it should use artifacts from the build directories for testing. The Python tests in gz-math behave like this, for example.
2. The existing prediction doesn't work when GZ_PYTHON_INSTALL_PATH is changed, perhaps by setting USE_SYSTEM_PATHS_FOR_PYTHON_INSTALLATION=ON.
3. When DESTDIR is used to direct make where to install files instead of CMAKE_INSTALL_PREFIX, the final target directory isn't known to ctest at all.
4. By overriding PYTHONPATH unconditionally, it's not possible to set PYTHONPATH manually prior to invoking ctest. Without resorting to using CMAKE_INSTALL_PREFIX or modifying the cmake, it's impossible to get the test to pass.

Note that DESTDIR is the installation root mechanism used in both Debian and RPM builds, but colcon uses CMAKE_INSTALL_PREFIX because DESTDIR is a Makefile concept and doesn't typically work on Windows.