Add t/lib/Mo and a bunch of tests

t/lib/Mo/is.t

@@ -0,0 +1,26 @@
+#!/usr/bin/perl
+
+BEGIN {
+   die "The PERCONA_TOOLKIT_BRANCH environment variable is not set.\n"
+      unless $ENV{PERCONA_TOOLKIT_BRANCH} && -d $ENV{PERCONA_TOOLKIT_BRANCH};
+   unshift @INC, "$ENV{PERCONA_TOOLKIT_BRANCH}/lib";
+};
+
+use strict;
+use warnings FATAL => 'all';
+use English qw(-no_match_vars);
+use Test::More;
+
+plan tests => 2;
+
+package Foo::is;
+use Mo qw(is);
+
+has 'stuff' => (is => 'ro');
+
+package main;
+
+my $f = Foo::is->new(stuff => 'foo');
+is $f->stuff, 'foo', 'values passed to constructor are successfully accepted';
+eval { $f->stuff('barbaz') };
+ok $@, 'setting values after initialization throws an exception';