JUnit reports error in case of parametrized tests

[mivallion]

Luatest writes reports with double quotes around the values of test parameters when they are strings, which breaks the XML format.

Test group example:

local g_positive_start_stages = t.group('touching_positive.start.stages', {
    { stage = const.TOUCHING.STAGE.COMPLETED },
    { stage = const.TOUCHING.STAGE.CANCELLED },
})

Result JUnit xml:

<testcase group="touching_positive.start.stages.stage:"COMPLETED" name="touching_positive.start.stages.stage:"COMPLETED".test_start" time="0.040">
</testcase>