produce valid xml for null-byte in test name

[maxgabut]

This solve #468.

A parameterized test name can be anything, and it can even include the null-byte.

It seems that the null-byte is not part of the valid character range for xml (see Character Range in xml), so we can not encode it. But nothing prevents us from representing it somehow, and this is what is proposed here.

With this PR, when we need to represent a null-byte in xml, we output \0 instead. This is not perfect, reversible, a parser would not return a null-byte when reading \0, but I believe it to be good enough for reports generation.

[hcoles]

Thanks!