internal/proxydatasource: increase test timeout

This test suite is flaking in CI, with a relatively short timeout of 5s.
Increase to 20s.

Change-Id: Ifb9095d8b453a68e756e4dbf87d80eb4cbecf6e8
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/245320
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

internal/proxydatasource/datasource_test.go

@@ -43,7 +43,7 @@ func setup(t *testing.T) (context.Context, *DataSource, func()) {
 		},
 	}
 	client, teardownProxy := proxy.SetupTestProxy(t, testModules)
-	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	ctx, cancel := context.WithTimeout(context.Background(), 20*time.Second)
 	return ctx, New(client), func() {
 		teardownProxy()
 		cancel()