tests/slurm-epilog: strengthen mount-boundary assertions and add pathname regressions #1409

Follow-up from #1408 review. The production fixes are merged; these are test-strength gaps only, in tests/slurm-epilog/run-tests.sh:

1. mount_case runs the rendered 42-lastuserjob-cleanup and discards its exit status. A cleanup that exits early (for example, a replacement stub that exit 17) still passes as long as the payload file was never reached.
2. assert_owned_mount_root only checks that the payload under the user-owned mount root still exists. It should also require positive sibling cleanup outside the mount, as the owned-parent and same-device bind cases already do, so the case cannot pass vacuously.
3. There are no regression cases for mount points containing *, ?, [, backslash or a trailing newline, with unrelated glob-lookalike siblings present (for example data1 next to a mount named data[1]). The escaping and printf -v decode added in Scope the Slurm epilog cleanup to the job that ended #1408 are only covered by out-of-tree checks today. Since the root-only mount cases skip on most developer machines, a variant that stubs awk to emit a synthetic mount record (as the maintainer review did) would let these run unprivileged.

No production code change is expected.