tests/iso-parser/run.sh: add ISO boot tool test harness

Generic test harness covering all ISO boot scripts:
- kexec-parse-boot.sh: 11 mock trees testing GRUB, syslinux, BLS, tab-indented,
  --- dash separators, deep paths, GRUB variables, loopback.cfg (INLINE/SOURCE)
- kexec-parse-bls.sh: BLS format configs via loader/entries/*.conf
- unpack_initramfs.sh: multi-segment cpio, kernel module detection,
  no-modules initrd (blank marker), ext4/fat/btrfs module verification
- kexec-iso-init.sh: boot_marker() and fmt_boot_target() extracted from
  kexec-select-boot.sh for per-initrd [OK]/[!]/blank display testing
- --with-isos mode mounts real ISO files via fuseiso, runs full pipeline
- Results: PASS/FAIL/SKIP counters with per-ISO row in matrix

Signed-off-by: Thierry Laurion <insurgo@riseup.net>

Author: tlaurion