added commandline and stdin tests using testscript (#28)

* added commandline and stdin tests using testscript

---------

Co-authored-by: Thomas von Dein <tom@vondein.org>

t/cmdlinecalc-lua.txtar

@@ -0,0 +1,16 @@
+exec testrpn -d -c test.lua 3 5 lower
+stdout '3\n'
+
+-- test.lua --
+function lower(a,b)
+    if a < b then
+        return a
+    else
+        return b
+    end
+end
+
+function init()
+    -- expects 2 args
+    register("lower", 2, "lower")
+end