21 cases through a stubbed sanderling: every exit path, the drift check,
a missing trace, a zero-byte trace, an empty glob and a truncated line.
Asserts the flags that reached the binary, not just the exit code.
Invoked as bash -eo pipefail -c, which is what a run: block does. Running
folio-run.sh itself under -e would kill it at the first non-zero
sanderling test, which is the exit code it exists to read.
