Makefile in the tests directory; ln command needs the "f" switch

**Describe the bug**
On a lark, I ran make test and got an error with the ln command. It needs an 'f' in the switch in order to replace any existing link. I checked the other Makefiles and did NOT find any others with this problem.

Here's a snippet of the Makefile; I added the 'f' in the '-sf' switch:

437 [ -f lz4 ] || ln -sf $(PRGDIR)/zstd lz4
438 [ -f unlz4 ] || ln -sf $(PRGDIR)/zstd unlz4

**To Reproduce**
Steps to reproduce the behavior:
1. Downloads data '...'
2. Run '...' with flags '...'
3. Scroll up on the log to '....'
4. See error

**Expected behavior**
The xz link in the tests directory should be successful and not generate an error message.

**Screenshots and charts**
Not needed.

**Desktop (please complete the following information):**
- OS: Fedora
- Version 42
- Compiler gcc (GCC) 15.0.1 20250325 (experimental)
- Flags [e.g. O2]
- Other relevant hardware specs [e.g. Dual-core]
- Build system Makefile

**Additional context**
Add any other context about the problem here.