Fix issue 20145 - Random unittest failures in std.datetime.stopwatch

[ghost]

Times on benchmarks are not generally comparable (think of the computer heavily swapping during the first benchmark while not doing so during the second). I removed three asserts, which cannot generally be asserted and replaced them with two asserts, which give weaker, but save upper bounds.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20145	normal	Random unittest failures inf std.datetime.stopwatch

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + phobos#7186"