tests: automake: keep the libs link at the final stage

Currently the static archive libdrmtest.la links against libdrm.la.
Only to have both added to the executables' LDADD. Simplify things a
bit, by doing the link in the final module (the executables/tests in
this case).

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>