unit test introduced:

User should create <test-name>-test.c file in the following manner:
> cat codegen/function/sum-test.c
extern void *resolve_symbol(const char*);

int
run_test() {
  int (*sum)(int, int) = resolve_symbol("kfun:sum");

  if (sum(2, 3) != 5) return 1;

  return 0;
}
and add <test-name> to the TESTS variable in the Makefile
to run tests:
> make clean run
Note: failing tests should return no zero values.

backend.native/tests/codegen/function/sum-test.c

@@ -0,0 +1,10 @@
+extern void *resolve_symbol(const char*);
+
+int
+run_test() {
+  int (*sum)(int, int) = resolve_symbol("kfun:sum");
+  
+  if (sum(2, 3) != 5) return 1;
+
+  return 0;
+}

backend.native/tests/codegen/function/sum.kt

@@ -0,0 +1 @@
+fun sum(a:Int, b:Int) = a + b

backend.native/tests/codegen/function/sum_func.kt

@@ -0,0 +1,3 @@
+fun foo():Int = 1
+//fun bar():Int = 2
+//fun sum():Int = foo() + bar()

backend.native/tests/codegen/function/sum_silly.kt

@@ -0,0 +1,5 @@
+fun sum(a:Int, b:Int):Int {
+ var c:Int
+ c = a + b
+ return c
+}