[cmake] core_add_test_library also add headers to IDE

[Rechi]

Motivation and Context

Header files for tests were missing in Visual Studio.

How Has This Been Tested?

cmake generation

Types of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the Code guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• I have read the CONTRIBUTING document
• I have added tests to cover my change
• All new and existing tests passed

[wsnipex]

looks good to me, @fetzerch ping

[fetzerch]

It should be fine, not hundert percent sure if GTEST_ADD_TESTS (in main cmakelists.txt) will be ok, but jenkins will tell - hopefully ;)

jenkins build this please

[Rechi]

any objections on this?