[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/dictionary-mismatch.sh

@@ -0,0 +1,29 @@
+#!/bin/sh
+
+source "$COMMON/platform.sh"
+
+set -e
+
+if [ false ]; then
+	for seed in $(seq 100); do
+		datagen -g1000 -s$seed > file$seed
+	done
+
+	zstd --train -r . -o dict0 -qq
+
+	for seed in $(seq 101 200); do
+		datagen -g1000 -s$seed > file$seed
+	done
+
+	zstd --train -r . -o dict1 -qq
+
+	[ "$($MD5SUM < dict0)" != "$($MD5SUM < dict1)" ] || die "dictionaries must not match"
+
+	datagen -g1000 -s0 > file0
+fi
+
+set -x
+zstd files/0 -D dicts/0
+zstd -t files/0.zst -D dicts/0
+zstd -t files/0.zst -D dicts/1 && die "Must fail" ||:
+zstd -t files/0.zst            && die "Must fail" ||: