Use upstream default ${libexecdir} now that Debian Policy allows /usr/libexec (via FHS 3.0)

debian/rules

@@ -66,7 +66,7 @@ endif
 
 override_dh_install:
 	rm -f debian/tmp/usr/lib/*/*.la
-	rm -f debian/tmp/usr/lib/installed-tests/libostree/*.la
+	rm -f debian/tmp/usr/libexec/installed-tests/libostree/*.la
 	:
 	# TODO: when someone documents how to test these (#824649) they
 	# should be installed in a new ostree-boot package: see
@@ -78,7 +78,7 @@ override_dh_install:
 	rm -f debian/tmp/lib/systemd/system/ostree-prepare-root.service
 	rm -f debian/tmp/lib/systemd/system/ostree-remount.service
 	rm -fr debian/tmp/usr/lib/dracut/modules.d/98ostree/
-	rm -f debian/tmp/usr/lib/libostree/grub2-15_ostree
+	rm -f debian/tmp/usr/libexec/libostree/grub2-15_ostree
 	rm -f debian/tmp/usr/lib/ostree/ostree-prepare-root
 	rm -f debian/tmp/usr/lib/ostree/ostree-remount
 	rm -f debian/tmp/usr/lib/tmpfiles.d/ostree-tmpfiles.conf