Fix WriteLargeKeyOrValue System.Text.Json test (#39857)
Fix WriteLargeKeyOrValue System.Text.Json test (#39857)

When the Utf8JsonWriter is disposed in the first part of the test, it flushes the WriteStartObject token to the ArrayBufferWriter, such that the output.WrittenCount is already 1 when the second part of the test starts, and then the Assert.Equal(0, output.WrittenCount) is doomed to fail.
ViktorHofer/corefx,ViktorHofer/corefx,wtgodbe/corefx,ericstj/corefx,ViktorHofer/corefx,ericstj/corefx,shimingsg/corefx,ericstj/corefx,shimingsg/corefx,shimingsg/corefx,ViktorHofer/corefx,wtgodbe/corefx,ericstj/corefx,ViktorHofer/corefx,ViktorHofer/corefx,shimingsg/corefx,ericstj/corefx,ericstj/corefx,shimingsg/corefx,shimingsg/corefx,wtgodbe/corefx,wtgodbe/corefx,shimingsg/corefx,ericstj/corefx,ViktorHofer/corefx,wtgodbe/corefx,wtgodbe/corefx,wtgodbe/corefx