Fix WriteLargeJsonToStreamWithoutFlushing test (dotnet#34914)

.NET 5.0.0-preview.1

Fix overaggressive CanBeMadeAtomic check for Set + Notone (dotnet#33409)

We're erroneously converting a set loop to be atomic when it's followed by a notone where the notone's character is in the set.  But if we for example have `[ab]*[^a]`, we can't make the loop atomic, because the `[ab]*` can actually give back something (a `b`) that the `[^a]` will match.  The fix is simply to delete the erroneous, overaggressive checks.