test(alerting): Improve coverage for custom alerting provider

alerting/provider/custom/custom_test.go

@@ -13,14 +13,24 @@ import (
 )
 
 func TestAlertProvider_IsValid(t *testing.T) {
+	t.Run("invalid-provider", func(t *testing.T) {
 		invalidProvider := AlertProvider{URL: ""}
 		if invalidProvider.IsValid() {
 			t.Error("provider shouldn't have been valid")
 		}
+	})
+	t.Run("valid-provider", func(t *testing.T) {
 		validProvider := AlertProvider{URL: "https://example.com"}
+		if validProvider.ClientConfig != nil {
+			t.Error("provider client config should have been nil prior to IsValid() being executed")
+		}
 		if !validProvider.IsValid() {
 			t.Error("provider should've been valid")
 		}
+		if validProvider.ClientConfig == nil {
+			t.Error("provider client config should have been set after IsValid() was executed")
+		}
+	})
 }
 
 func TestAlertProvider_Send(t *testing.T) {