Improve timing related tests

[jnyrup]

This PR consists of two commits related to timing:

1. JavaScript's Date.now() is documented as

The Date.now() static method returns the number of milliseconds elapsed since the epoch, which is defined as the midnight at the beginning of January 1, 1970, UTC.

C# DateTime.Now on the other hand is documented as:

Gets a DateTime object that is set to the current date and time on this computer, expressed as the local time.

If running a test while changing from/to DST, the difference between two calls to DateTime.Now is no longer only an approximation of the time to run some code, but also the difference in UTC offsets.
By changing usages of DateTime.Now to DateTime.UtcNow we remove that "race-condition" and brings the intention closer to the puppeteer tests.

The second commit includes idea from #2090 (comment) to avoid silently ignoring exceptions in some tests.
@amaitland I did see your wish for a more elegant solution, but do you agree that including the less pretty solution for now is better than keeping status quo?

[amaitland]

but do you agree that including the less pretty solution for now is better than keeping status quo?

@jnyrup Absolutely 👍 I've rewritten a couple of cases (#2096) where ContinueWith wasn't necessary, that's as far as I've got so far.