tests: don't hardcode systemctl path

Get it from type -P instead, to support --enable-split-usr.

Makefile.am

@@ -1433,7 +1433,7 @@ EXTRA_DIST += \
 	test/c.service \
 	test/daughter.service \
 	test/d.service \
-	test/end.service \
+	test/end.service.in \
 	test/e.service \
 	test/f.service \
 	test/grandchild.service \

test/end.service.in

@@ -3,4 +3,4 @@ Description=End the test
 After=testsuite.service
 
 [Service]
-ExecStart=/usr/bin/systemctl poweroff --no-block
+ExecStart=@SYSTEMCTL@ poweroff --no-block

test/test-functions

@@ -293,7 +293,8 @@ install_terminfo() {
 }
 
 setup_testsuite() {
-    cp $TEST_BASE_DIR/{testsuite.target,end.service} $initdir/etc/systemd/system/
+    cp $TEST_BASE_DIR/testsuite.target $initdir/etc/systemd/system/
+    sed "s#@SYSTEMCTL@#$(type -P systemctl)#g" $TEST_BASE_DIR/end.service.in > $initdir/etc/systemd/system/end.service
 
     mkdir -p $initdir/etc/systemd/system/testsuite.target.wants
     ln -fs $TEST_BASE_DIR/testsuite.service $initdir/etc/systemd/system/testsuite.target.wants/testsuite.service