[test] Add new CLI testing platform

Adds the new CLI testing platform that I'm proposing.
See the added `README.md` for details.

tests/cli-tests/dictionaries/setup_once

@@ -0,0 +1,24 @@
+#!/bin/sh
+
+set -e
+
+source "$COMMON/platform.sh"
+
+
+mkdir files/ dicts/
+
+for seed in $(seq 50); do
+	datagen -g1000 -s$seed > files/$seed
+done
+
+zstd --train -r files -o dicts/0 -qq
+
+for seed in $(seq 51 100); do
+	datagen -g1000 -s$seed > files/$seed
+done
+
+zstd --train -r files -o dicts/1 -qq
+
+cmp dicts/0 dicts/1 && die "dictionaries must not match!"
+
+datagen -g1000 > files/0