TimeVal: fix init regression, add tests

Closes #703 (closed)

It was stupid, I did not really realize how we export TimeVal, and that all additional fancy-constructors are added in Python layer.

In cpp layer there is only one constructor: with no agrs. However, somehow it was compiling (but did not work, actually). And, what was even more stupid, the py::init<float>() was narrowing down to py::init<int, int, int>, where the last two args were 0. And here Windows build faild due to MSVS is more strict wrt GCC and Clang.

Now everything is fixed and covered with test.

Note for the future: no MR without tests anymore!