Add ament_cmake_auto_gtest

[cho3]

I use ament_cmake_auto a lot and ament_cmake_gtest a lot, so I find myself writing a lot of stuff I don't have to for the libraries and executables.

Here is a copy/paste Frankenstein solution between ament_auto_add_executable and ament_add_gtest to start the conversation.

I have tested this locally on a very limited use case.

Signed-off-by: Christopher Ho christopherj.ho@gmail.com

[sloretz]

@mjcarroll friendly ping :)

[ivanpauno]

@mjcarroll friendly ping

[KeisukeShima]

@cho3 Hi,
Thank you for your wonderful pull request. I've seen the changes, but it looks like there's a problem with how to use cmake_parse_arguments.

The documentation is here.
https://cmake.org/cmake/help/v3.6/command/cmake_parse_arguments.html

My suggestions are:

  cmake_parse_arguments(ARG
    "WIN32;MACOSX_BUNDLE;EXCLUDE_FROM_ALL;NO_TARGET_LINK_LIBRARIES;SKIP_LINKING_MAIN_LIBRARIES;SKIP_TEST"
    "RUNNER;TIMEOUT;WORKING_DIRECTORY;DIRECTORY"
    "APPEND_ENV;APPEND_LIBRARY_DIRS;ENV"
    ${ARGN})