add unit tests for media key parsing

We've had at least a couple media key parsing bugs. Let's get a *little*
more confidence that this code works :)

src/service/media.rs

@@ -14,6 +14,7 @@ mod data;
 
 pub(crate) use data::Data;
 
+#[derive(Debug, Eq, PartialEq)]
 pub(crate) struct FileMeta {
     // This gets written to the database but we no longer read it
     //