btrfs: tests: add helper for error messages and update them

The test failures are not clearly visible in the system log as they're
printed at INFO level. Add a new helper that is level ERROR. As this
touches almost all strings, I took the opportunity to unify them:

- decapitalize the first letter as there's a prefix and the text
  continues after ":"
- glue strings split to more lines and un-indent so they fit to 80
  columns
- use %llu instead of %Lu
- drop \n from the modified messages (test_msg is left untouched)

Signed-off-by: David Sterba <dsterba@suse.com>