[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/mtime.sh

@@ -0,0 +1,13 @@
+source "$COMMON/platform.sh"
+
+MTIME="stat -c %Y"
+case "$UNAME" in
+    Darwin | FreeBSD | OpenBSD | NetBSD) MTIME="stat -f %m" ;;
+esac
+
+assertSameMTime() {
+    MT1=$($MTIME "$1")
+    MT2=$($MTIME "$2")
+    echo MTIME $MT1 $MT2
+    [ "$MT1" = "$MT2" ] || die "mtime on $1 doesn't match mtime on $2 ($MT1 != $MT2)"
+}