x11-toolkits/qt5-declarative: fix testlib-build-failures

Since Qt5 testlib tries to be a build-dependency only when
needed, and since we build the QML bits without installing
the corresponding test-tools, declarative had the following
problem:

• without testlib installed, would build fine
• with testlib installed, would assume the QML test bits are available, and then fail in the build.

This is basically our split-up-packaging without subpackages
shooting us in the foot; fix the build by removing one more
place where testlib leads to the building-of-tests. (The existing
patch files/patch-src_src.pro does something similar)

PR: 257102 260433
Reported by: Frank Reisert, Philipp Ost

(cherry picked from commit 9224581a101ea6aa9cfc84002ec344d49b1ee259)