xunit: rename testsuite attribute skip to skipped

Currently, xunit output looks like:

    <testsuite ... skip="0">

However, all examples of JUnit results (and also some other xUnits that look similar) either ignore this attribute completely, or use `skipped` instead of skip. So the xunit looks like this:

    <testsuite ... skipped="0">

This small issue has one bad inpact: Bamboo JUnit parser will not consider the file to be valid when `skip` is used instead of `skipped`. So I wrote this patch to change it to `skipped`. I did not find any xunit specification that would state that the attribute should be `skip` while `skipped` is used quite often in existing JUnit files.

However, this change might have some impact to existing users of Robot Framework xunit output, who might already have their custom parsers relying on `skip`. But are there any?