Add an integration test suite using the file tape backend

[matejk]

Summary

A shell-based integration test suite (14 tests) driven by make check, mounting a file-backend volume from the build tree — no installation or tape hardware needed. Tests need a Linux host with /dev/fuse and are skipped elsewhere; tests/run-in-docker.sh runs the suite in an Ubuntu container for development on non-Linux hosts. A CI workflow runs the suite on every push.

Coverage: mount/unmount, data roundtrips, truncate, rename, directory operations, xattrs, symlinks, large directories, a tar roundtrip, rename flags (RENAME_NOREPLACE/RENAME_EXCHANGE) and ftruncate via a small C helper, inode stability across remounts (verifies use_ino/index-UID passthrough), FUSE request sizes, direct_io, readdirplus, and content verification of writes larger than the tape block size.

Relationship to the other PRs

• The first commit duplicates the arch_open permissions fix from Fix file permissions passed to open(2) by the POSIX arch_open #612: the file tape backend created files with mode 0200, so formatting and mounting as a regular user — which this suite and its CI do — failed with EDEV_RW_PERM. The duplicate keeps this PR's CI green standalone; git drops it whichever merges first.

• t/13-large-write-integrity.sh is the regression test for the scheduler data loss (silent data corruption on ARM64? #591): single multi-block writes of random data, content-compared while mounted and after a remount. It passes once Fix data loss on writes larger than the tape block size #607 (or Add libfuse 3 support with libfuse 2 fallback #603, which carries the same fix) is merged.

• t/10–t/12 are FUSE-version aware: on libfuse 2 the request-size test expects the big_writes limit and the readdirplus prefill is not asserted; the strict expectations activate with the FUSE 3 port (Add libfuse 3 support with libfuse 2 fallback #603). All three pass on both APIs. Add libfuse 3 support with libfuse 2 fallback #603 ships the same three scripts with its feature commits; the contents are identical, so the merge is clean in either order.

• The test list is computed from tests/t/ (wildcard in automake), and the harness locates build artifacts in both the autotools and CMake layouts, so the same scripts run under make check and the ctest registration of Add a CMake build alongside autotools #606 — no registration conflicts in any merge order.

Verification

All 14 tests pass on Ubuntu against both fuse2 and fuse3 builds (with #603 merged alongside), via make check and ctest.