Set libexecdir to /usr/lib

The conservative default is the same as ${libdir}, i.e.
/usr/lib/MULTIARCH, but there's no real point in multiarch for most
libexec binaries (in particular, for our tests) and it makes finding
the tests a little harder.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Justin Kim <justin.kim@collabora.com>
Differential Revision: https://phabricator.apertis.org/D6893

debian/liblightwood-tests.install

-usr/lib/*/installed-tests/liblightwood-1
+usr/lib/installed-tests/liblightwood-1

debian/rules

@@ -13,6 +13,7 @@ override_dh_auto_configure:
 		--enable-always-build-tests \
 		--enable-documentation \
 		--enable-installed-tests \
+		--libexecdir='$${prefix}/lib' \
 		$(NULL)
 
 override_dh_install: