test-amalgamation: use a single cat command

test-amalgamation: use a single cat command

Use the list of prerequisites instead of listing those files manually,
this way they will never fall out of sync.

Also update the amalgamation example to use a single cat command.

Fixes: a7e8d394 ("[amalgamation] add test")
Fixes: b192c86b ("[amalgamation] lz4frame.c")