Add a test case for the custom mutator

- Update the Makefile in examples/custom_mutators
- Add a test program for testing the custom mutator
- Update test.sh for testing the custom mutator
- [TODO] Update the result checking criterias of the custom mutator in
test.sh

examples/custom_mutators/Makefile

@@ -1,2 +1,7 @@
-all:
+all: libexamplemutator.so
+
+libexamplemutator.so:
 	$(CC) $(CFLAGS) -fPIC -shared -g -I ../../include example.c -o libexamplemutator.so
+
+clean:
+	rm -rf libexamplemutator.so