[CI] Hook cli-tests up to CI

Add cli-tests to `make test`. This adds a `python3` dependency to `make
test`, but not `make check`. We could make this dependency optional by
skipping the tests if `python3` is not present.

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

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

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

@@ -1,4 +1,4 @@
-+ zstd --train
+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#