tests: introduce is_num_children test

This makes it clearer what the tests are actually doing and kills quite
a bit of useless repetitions

testcases/lib/i3test/Test.pm

@@ -0,0 +1,63 @@
+package i3test::Test;
+
+use base 'Test::Builder::Module';
+
+our @EXPORT = qw(is_num_children);
+
+my $CLASS = __PACKAGE__;
+
+=head1 NAME
+
+i3test::Test - Additional test instructions for use in i3 testcases
+
+=head1 SYNOPSIS
+
+  use i3test;
+
+  my $ws = fresh_workspace;
+  is_num_children($ws, 0, 'no containers on this workspace yet');
+  cmd 'open';
+  is_num_children($ws, 1, 'one container after "open"');
+
+  done_testing;
+
+=head1 DESCRIPTION
+
+This module provides convenience methods for i3 testcases. If you notice that a
+certain pattern is present in 5 or more test cases, it should most likely be
+moved into this module.
+
+=head1 EXPORT
+
+=head2 is_num_children($workspace, $expected, $test_name)
+
+Gets the number of children on the given workspace and verifies that they match
+the expected amount of children.
+
+  is_num_children('1', 0, 'no containers on workspace 1 at startup');
+
+=cut
+
+sub is_num_children {
+    my ($workspace, $num_children, $name) = @_;
+    my $tb = $CLASS->builder;
+
+    my $con = i3test::get_ws($workspace);
+    $tb->ok(defined($con), "Workspace $workspace exists");
+    if (!defined($con)) {
+        $tb->skip("Workspace does not exist, skipping is_num_children");
+        return;
+    }
+
+    my $got_num_children = scalar @{$con->{nodes}};
+
+    $tb->is_num($got_num_children, $num_children, $name);
+}
+
+=head1 AUTHOR
+
+Michael Stapelberg <michael@i3wm.org>
+
+=cut
+
+1