test_get_chained_prefix_path when AMENT_PREFIX_PATH is set

[cottsay]

If AMENT_PREFIX_PATH is set when colcon-core tests are invoked and the necessary colcon extensions are also available, the test will fail because of the unexpected additional paths.

I think it would be prudent to either:
a) Make the test assert a change in paths, rather than an explicit list of paths
b) Explicitly set the list of available prefix path extensions to avoid the unexpected ones