pkg/index: Rename audiotag to mediatag.

Also fix up keys and values and add tests.

Change-Id: I7e6c5c4315705442e3517456f2ba16419af49f2f
make.go: Mirror .mp3 testdata.

Change-Id: Iab40eb1cc7799fab1b499c9a0354114a65f0b45a
test: add TODO to simplify