add test code

ci/app/go_test.go

@@ -0,0 +1,26 @@
+package main
+
+import (
+    "net/http"
+    "net/http/httptest"
+    "strings"
+    "testing"
+)
+
+
+func TestHealthMessage(t *testing.T) {
+    wr := httptest.NewRecorder()
+    req := httptest.NewRequest(http.MethodGet, "/health", nil)
+
+    healthMessage(wr, req)
+    if wr.Code != http.StatusOK {
+        t.Errorf("got HTTP status code %d, expected 200", wr.Code)
+    }
+
+    if !strings.Contains(wr.Body.String(), `{"health":"OK"}`) {
+        t.Errorf(
+            `response body "%s" does not contain "health OK"`,
+            wr.Body.String(),
+        )
+    }
+}