testsuite: cover more path/file-operation code (syscall.c, util1.c, delete.c)

testsuite: cover more path/file-operation code (syscall.c, util1.c, delete.c)

Target previously-uncovered functions in the path/file-operation files the
resolver restructure touches, confirmed hit under coverage:

  preallocate   --preallocate (syscall.c do_fallocate) and sparse hole-punching
                via --preallocate --sparse and --inplace --sparse (do_punch_hole),
                on a file several levels deep.
  fuzzy-basis   --fuzzy basis selection with similar-named candidates and no
                exact match, so the generator scores them (util1.c fuzzy_distance).
  delete-deep   add a --backup --delete case so removing an extraneous
                backup-suffixed file consults delete.c is_backup_file.

preallocate probes --preallocate support up front and skips where it is
unavailable: macOS, the *BSDs and Solaris build without fallocate/posix_fallocate
(and FALLOC_FL_PUNCH_HOLE is Linux-only), and reject the option outright. It runs
on Linux and Cygwin. fuzzy-basis and delete-deep are plain local transfers with
no skips. All green on master and under --protocol=29/30.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>