testing: Add TestCheckResourceAttrSet helper

This commit adds a TestCheckFunc which ensures that a value is set for a
given name/key combination. It is primarily useful for ensuring that
computed values are set where it is not possible to know the expected
value ahead of time.

helper/resource/testing.go

@@ -520,6 +520,31 @@ func ComposeTestCheckFunc(fs ...TestCheckFunc) TestCheckFunc {
 	}
 }
 
+// TestCheckResourceAttrSet is a TestCheckFunc which ensures a value
+// exists in state for the given name/key combination. It is useful when
+// testing that computed values were set, when it is not possible to
+// know ahead of time what the values will be.
+func TestCheckResourceAttrSet(name, key string) TestCheckFunc {
+	return func(s *terraform.State) error {
+		ms := s.RootModule()
+		rs, ok := ms.Resources[name]
+		if !ok {
+			return fmt.Errorf("Not found: %s", name)
+		}
+
+		is := rs.Primary
+		if is == nil {
+			return fmt.Errorf("No primary instance: %s", name)
+		}
+
+		if val, ok := is.Attributes[key]; ok && val != "" {
+			return nil
+		}
+
+		return fmt.Errorf("%s: Attribute '%s' expected to be set", name, key)
+	}
+}
+
 func TestCheckResourceAttr(name, key, value string) TestCheckFunc {
 	return func(s *terraform.State) error {
 		ms := s.RootModule()