Use WIFEXITED() and WEXITSTATUS() on Unix system() exit code

(ensures Odin run returns correct exit code of built executable)
Adds test "tests/core/os/test_core_os_exit.odin" (Unix only)

tests/core/os/test_core_os_exit.odin

@@ -0,0 +1,10 @@
+// Tests that Odin run returns exit code of built executable on Unix
+// Needs exit status to be inverted to return 0 on success, e.g.
+// $(./odin run tests/core/os/test_core_os_exit.odin && exit 1 || exit 0)
+package test_core_os_exit
+
+import "core:os"
+
+main :: proc() {
+	os.exit(1)
+}