test(acceptance): Wait for fonts to be loaded for snapshots (#20255)

Check that `document.fonts.status === "loaded"` before snapshotting. This is hard to repro, so will monitor snapshots to see if this actually works. Otherwise we may need to poll for the font in question to be loaded.

This is attempting to fix a flakey visual snapshot that occurs in getsentry where Rubik (specifically with font weight 400) has not loaded when snapshot is taken.

src/sentry/utils/pytest/selenium.py

@@ -263,6 +263,14 @@ class Browser(object):
 
         return self
 
+    def wait_for_fonts_loaded(self, timeout=10):
+        wait = WebDriverWait(self.driver, timeout)
+        wait.until(
+            lambda driver: driver.execute_script("""return document.fonts.status === 'loaded'""")
+        )
+
+        return self
+
     def blur(self):
         """
         Find focused elements and call blur. Useful for snapshot testing that can potentially capture
@@ -303,8 +311,9 @@ class Browser(object):
                 time.sleep(1)
 
         if os.environ.get("VISUAL_SNAPSHOT_ENABLE") == "1":
-            # wait for images to be loaded
+            # wait for external assets to be loaded
             self.wait_for_images_loaded()
+            self.wait_for_fonts_loaded()
 
             # Note: below will fail if these directories do not exist