This drops testfiles.ReadOrDie and updated testfiles.Exists to return an
error, forcing the caller to decide whether to call framework.Fail or do
something else.
It makes for a slightly less friendly API, but also means the package is
decoupled from framework again, as per the comments at the top of the
file
Most of these could have been refactored automatically but it wouldn't
have been uglier. The unsophisticated tooling left lots of unnecessary
struct -> pointer -> struct transitions.
