Wrap tap subtests with pcall

For example, when we write test:test with subtests, for example:

test:test("Chunked index with filter", function(test)
    local res = mock.run( test, 3, 'correct', 'data')
    test:is(res.ctl, 'customer', "Controller found")
    test:is(#res.index, 1, "Key detected")
    test:is(res.filter, "pers_data.name_first", "Filter detected")
end)

If data res is nil, the we'll get exception

[001] +2017-08-30 15:25:13.512 [22183] main/101/single.test.lua F> single.test.lua:100: attempt to index global 'res' (a nil value)

And tests'll be stopped. Better to wrap it with pcall/xpcall, catch exception and provide it (with stacktrace) in tap metadata.

Begin at the test:test subtest entry point and inspect how TAP metadata is emitted. Reproduce the nil res failure from the example, then trace pcall/xpcall handling and verify that the exception and stack trace are reported as TAP metadata without stopping later subtests.