Add test for print.google_font

tests/testthat/google-font-print.txt

+Family:  Josefin Sans 
+Weights: 400,400i,600i,700 
+URL:     https://fonts.googleapis.com/css?family=Josefin+Sans:400,400i,600i,700

tests/testthat/test-utils.R

+test_that("printing google_font works", {
+  expect_known_output(
+    print(google_font("Josefin Sans", "400", "400i", "600i", "700")),
+    "google-font-print.txt"
+  )
+})