[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/dict-builder/no-inputs

@@ -0,0 +1,3 @@
+#!/bin/sh
+set -x
+zstd --train

tests/cli-tests/dict-builder/no-inputs.exit

@@ -0,0 +1 @@
+14

tests/cli-tests/dict-builder/no-inputs.stderr.exact

@@ -0,0 +1,5 @@
++ zstd --train
+!  Warning : nb of samples too low for proper processing ! 
+!  Please provide _one file per sample_. 
+!  Alternatively, split files into fixed-size blocks representative of samples, with -B# 
+Error 14 : nb of samples too low