[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/common/format.sh

@@ -0,0 +1,19 @@
+#!/bin/sh
+
+source "$COMMON/platform.sh"
+
+zstd_supports_format()
+{
+	zstd -h | grep > $INTOVOID -- "--format=$1"
+}
+
+format_extension()
+{
+	if [ "$1" = "zstd" ]; then
+		printf "zst"
+	elif [ "$1" = "gzip" ]; then
+		printf "gz"
+	else
+		printf "$1"
+	fi
+}