insane.conf: add new libexec test

insane.conf: add new libexec test

This tests for /usr/libexec as we are moving things to /lib/.
the test is ignored if the distro defaults to /usr/libexec.

Currently this test will be disabled by default since the current
value of ${libexecdir} is "/usr/libexec".  Also this tests needs
to be enabled in the WARN_QA list.

[YOCTO #2915]

(From OE-Core rev: 4c60c2779dde6962f342f9c9b61713cf2d3a564c)

Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>