Implement test for file opening with false name

tests/test_currentCustomerAccountBalance.c

@@ -51,6 +51,13 @@ void test_checkFileOpen(void) {
     fclose(file);
 }
 
+void test_failOpenFile(void) {
+    FILE *file = fopen("false_file_name", "r");
+
+    TEST_ASSERT_FALSE(file);
+
+}
+