add tests for error paths, option coverage, and fix existing test bugs

[abhinavagarwal07]

• Fix tst_open_unlink: errno assertion was dead code (inside pytest.raises block)
• Fix tst_passthrough: mnt_name pointed to src_dir, making stat comparison a no-op
• Add tst_open_writeonly_read, tst_access, tst_mkdir_exist, tst_readdir_repeated, tst_rename_sibling
• Strengthen existing tests: listdir checks on rename, truncate-to-zero, symlink cleanup, nlink after hardlink removal
• Add standalone tests for disable_hardlink, follow_symlinks, and direct_io mount options
• Compare portable stat fields in tst_passthrough (avoids st_dev/st_ino mismatch across FUSE)