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

test/test-custom-mutator.c

@@ -0,0 +1,20 @@
+/**
+ * Reference: https://github.com/bruce30262/libprotobuf-mutator_fuzzing_learning/blob/master/4_libprotobuf_aflpp_custom_mutator/vuln.c
+ */
+
+#include <stdio.h>
+#include <string.h>
+#include <math.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+int main(int argc, char *argv[])
+{
+    char str[100]={ };
+    read(0, str, 100);
+    int *ptr = NULL;
+    if( str[0] == 'P') {
+        *ptr = 123;
+    }
+    return 0;
+}