Add 12 remaining 51media sytests

[ShadowJonathan]

This will add allsome remaining 51media sytests, excluding the ones ignored in #276;

./tests/51media/01unicode.pl
X Alternative server names do not cause a routing loop
  Can download specifying a different Unicode file name
  Can download with Unicode file name locally
  Can download with Unicode file name over federation
  Can upload with Unicode file name

./tests/51media/03ascii.pl
  Can download file '$filename'
  Can download specifying a different ASCII file name
  Can fetch images in room
  Can send image in room message
  Can upload with ASCII file name

./tests/51media/10thumbnail.pl
  POSTed media can be thumbnailed
  Remote media can be thumbnailed

./tests/51media/20urlpreview.pl
X Test URL preview

./tests/51media/30config.pl
  Can read configuration endpoint

Edit: Tests prefixed with X are removed after review.

This also adds a SplitMxc small helper function to client, for code dedup.

[ShadowJonathan]

See #278, all JSON keys are returned floats, apparantly.

[kegsay]

This needs a bit of work, but overall the code is excellent. The main points:

• Remove the tests which spin up web servers.
• Use image/png instead of a homebrew validator.
• Upgrade complement to 1.16 and use the embed package.

[kegsay]

Looks amazing, thank you!