Log files never created for long test names

[asomsiko]

Using gtest-parallel in projects with long test names (e.g. https://github.com/openvinotoolkit/openvino) I found it does not create log files in --output_dir for the tests exceed OS filename length limits. This blocks using gtest-parallel in Continuous Integration because logs are not available for issues analysis.

There was an issue tackling the similar problem but still not making logs available #57

[asomsiko]

@pbos I've drafted a fix in #76. Please share your thoughts there.

The fix is relying on mkstemp unique names generation and still adding test name as a prefix to a log name.