internal/zstd: use dynamic path resolution for zstd in tests
Abstract the hardcoded '/usr/bin/zstd' paths in fuzz and unit tests
to support systems where zstd may be installed at different locations.
The `findZstd` function uses `exec.LookPath` to locate the binary,
enhancing test portability.
Fixes #64000
Change-Id: I0ebe5bbcf3ddc6fccf176c13639ca9d855bcab87
GitHub-Last-Rev:
c4dfe1139bdc2f4f3200f80b314a02b5df5cd995
GitHub-Pull-Request: golang/go#64002
Reviewed-on: https://go-review.googlesource.com/c/go/+/540522
Reviewed-by: Klaus Post <klauspost@gmail.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Bryan Mills <bcmills@google.com>