playTests: use ln -sf for the xz/lzma symlinks

[deadcaf3]

Fixes #4431.

tests/playTests.sh creates ./xz, ./unxz, ./lzma and ./unlzma with ln -s and only removes them at the end of the xz block. The script runs under set -e, so a run that fails or is interrupted inside that block leaves the links in tests/, and every later run aborts with:

ln: ./xz: File exists

make clean does not remove them. This change uses ln -sf, as the script already does for the zstdcat and zcat links.

Testing

• Reproduced with make check: forced one command in the xz block to fail, reran, and got the error above.
• With this change the same rerun passes and the links are cleaned up. A run from a clean tree also passes.
• Run on macOS with xz support enabled. I also checked that GNU coreutils ln -sf replaces a stale or dangling link the same way.

Notes

• The issue quotes the lz4/unlz4 links in tests/Makefile. Those are guarded by [ -f lz4 ] || and did not fail when I tested them with stale links, so I left them unchanged. Happy to add -f there too if preferred.
• The issue has no log, so I can't tell what made the reporter's first run fail. This only stops the leftover links from breaking later runs.