Explicitly install before testing

Turns out that dehbelper does not install the package on its own so
we have to deal with it manually.

packaging/debian/rules

 	make install PREFIX=/usr DESTDIR=debian/tmp
 
 override_dh_auto_test:
+	make install PREFIX=/usr DESTDIR=debian/tmp
 	make test \
 		SHELLFU_PATH=$(shell pwd)/debian/tmp/usr/share/shellfu/include-bash