[FIX] tests: ffmpeg don't use the right path for screencast

Screencasts were saved in the right directory (screencasts_frames_dir)
but the given path for ffmpeg was wrong (screencasts_dir) and it wasn't
possible to read theses files to make the video.
After this commit, we use the screencast_frames folder to read files.

Steps to reproduce:
Execute Odoo with these additional command:
--screencasts "folder_of_destination" --test-enable (with a failed JS test to produce a screencast)

X-original-commit: 0c0b7a656388d65de15796f07a3f4cc93028f1d7

odoo/tests/common.py

@@ -984,7 +984,7 @@ class ChromeBrowser():
 
         if ffmpeg_path:
             framerate = int(len(self.screencast_frames) / (self.screencast_frames[-1].get('timestamp') - self.screencast_frames[0].get('timestamp')))
-            r = subprocess.run([ffmpeg_path, '-framerate', str(framerate), '-i', '%s/frame_%%05d.png' % self.screencasts_dir, outfile])
+            r = subprocess.run([ffmpeg_path, '-framerate', str(framerate), '-i', '%s/frame_%%05d.png' % self.screencasts_frames_dir, outfile])
             self._logger.runbot('Screencast in: %s', outfile)
         else:
             outfile = outfile.strip('.mp4')