Bug 891224 - sidebar tests fail: check for getAttribute("checked")=='false' rather than empty string

Uniform the sidebar testcases for `getAttribute('checked')` where in some platform returns `false` and in others an empty string (in case of `false` the attribute is just removed).
See: https://bugzilla.mozilla.org/show_bug.cgi?id=894809
Merge pull request #1143 from ZER0/sidebar/891224

Bug 891224 - sidebar tests fail: check for getAttribute("checked")=='false' rather than empty string r=@ZER0