add CMake function ament_add_test_label()

[dirk-thomas]

As syntactic sugar for ros2/ros2#900.

[sloretz]

The code LGTM; though personally I would more likely call this once

set_tests_properties(testname PROPERTIES LABELS "xfail;etc")

than append multiple labels with

ament_add_test_label(testname xfail)
ament_add_test_label(testname etc)

since I'll forget what it means in the future, and it's fewer steps to google set_tests_properities() than it is to google ament_add_test_label() followed by set_tests_properites().

[dirk-thomas]

though personally I would more likely call this once

But there are many cases were your CMake code in a package doesn't control the test target creation, e.g. https://github.com/ament/ament_index/blob/bfb1aa6f9f958561db616a9fcb84ca9e81e7fef3/ament_index_cpp/CMakeLists.txt#L44

That test target already has a label - gtest. In other cases like the linters the test has labels like linter;uncrustify.

If you set your own label on these tests using set_tests_properties() you will overwrite the existing labels.

[dirk-thomas]

than append multiple labels with

ament_add_test_label(testname xfail)
ament_add_test_label(testname etc)

You can also pass multiple labels to the function: ament_add_test_label(testname xfail etc).