Genex: Add a test for the LINK_ONLY expression.

Commit 239b0c6b (Don't add invalid content to static lib
INTERFACE_LINK_LIBRARIES., 2013-10-20) extended a test which
excercised the logic of the LINK_ONLY generator expression.

Commit ef10b87c (CMP0022: Plain target_link_libraries must populate
link interface, 2013-11-02) removed the instance of LINK_ONLY which
was excercised by that test.

Add a new test which excercises the other instance of LINK_ONLY by
setting the CMP0022 policy to NEW and consuming the contents of the
INTERFACE_LINK_LIBRARIES target property.
Merge topic 'test-LINK_ONLY-expression' into next

53b10fd Genex: Add a test for the LINK_ONLY expression.