[Test] Allow passing time_start as a keyword argument to DefaultTestSet

[giordano]

#59374 made time_start a const but now there's no way to instantiate a DefaultTestSet with a custom time_start without passing also all other fields.

[giordano]

I initially thought this would have been necessary for JuliaTesting/ParallelTestRunner.jl#21, but I figured out how to rearrange the code in the package not to need this. Still, I think this may be useful regardless of that issue, it's still impossible to pass a custom time_start in case someone needs to construct test set object after it has started.