Override SATURNIN_LIBEXEC in build-time tests

Otherwise tests can't pass, especially not on clean machine.

packaging/debian/rules

 	make install PREFIX=/usr DESTDIR=TEST
 	make test \
 		PATH="$(shell pwd)/TEST/usr/bin:$(PATH)" \
+		SATURNIN_LIBEXEC="$(shell pwd)/TEST/usr/libexec/saturnin-demo" \
 		SHELLFU_PATH="$(shell pwd)/TEST/usr/share/shellfu/include-bash"

packaging/template.spec

 %check
 make test \
     PATH="%{buildroot}/%{_bindir}:$PATH" \
+    SATURNIN_LIBEXEC="%{buildroot}/%{_libexecdir}/%{name}" \
     SHELLFU_PATH="%{buildroot}/%{sfincb}"
 
 %files

tests/TF_HEADER

 echo "version: $(saturnin-demo -V)"
 echo "binary: $(which saturnin-demo)"
 echo "SHELLFU_PATH: ${SHELLFU_PATH:-null}"
+echo "SATURNIN_LIBEXEC: ${SATURNIN_LIBEXEC:-null}"
 echo "shellfu-modules:"
 echo "    saturnin_demo_greet: $(shellfu _select_mfile saturnin_demo_greet)"
 echo "saturnin-version: $(saturnin-demo --saturnin-get-saturnin-version)"