feat: add linking tests

tests/programs/negr_missing_lib.fzs

@@ -0,0 +1,10 @@
+extern "jopalib"
+{
+  type JopaLibType handle;
+
+  fn jopalib(int a, str b) JopaLibType;
+}
+
+JopaLibType a = jopalib(42, "jopa");
+printline(a);
+

tests/programs/pos_math_lib.fzs

@@ -0,0 +1,8 @@
+extern "math" {
+  fn double_num(int num) int;
+}
+
+int initial = 42;
+int result = double_num(initial);
+printline($"initial value: {initial}, result of external function: {result}");
+

tests/programs/pos_renamed_lib.fzs

@@ -0,0 +1,8 @@
+extern "math" {
+  fn dnum(int num) int = "double_num";
+}
+
+int initial = 42;
+int result = dnum(initial);
+printline($"initial value: {initial}, result of external function: {result}");
+