[cli-tests] Fix zstd symlinks

The zstd symlinks, notably `zstdcat`, weren't working as expected
because only the `tests/cli-tests/bin/zstd` wrapper was symlinked. We
still invoked `zstd` with the name `zstd`. The fix is to create a
directory of zstd symlinks in `tests/cli-tests/bin/symlinks` for each
name that zstd recognizes. And when `tets/cli-tests/bin/zstd` is
invoked, it selects the correct symlink to call.

See the test `zstd-cli/zstdcat.sh` for an example of how it would work.

tests/cli-tests/zstd-symlinks/setup

@@ -0,0 +1,6 @@
+#!/bin/sh
+set -e
+
+println "hello" > hello
+println "world" > world
+zstd hello world

tests/cli-tests/zstd-symlinks/zstdcat.sh

@@ -0,0 +1,12 @@
+#!/bin/sh
+set -e
+
+# Test zstdcat symlink in bin/
+zstdcat hello.zst
+zstdcat hello.zst world
+zstdcat hello world.zst
+zstdcat hello.zst world.zst
+
+# Test local zstdcat symlink
+ln -s $(which zstd) ./zstdcat
+./zstdcat hello.zst

tests/cli-tests/zstd-symlinks/zstdcat.sh.stdout.exact

@@ -0,0 +1,8 @@
+hello
+hello
+world
+hello
+world
+hello
+world
+hello