Reduce inner and outer loop test time for S.T.Json (dotnet/corefx#35776)

Reduce inner and outer loop test time for S.T.Json (dotnet/corefx#35776)

* Reduce inner and outer loop test time for S.T.Json

* Remove redundant sequence slicing, only compare the SequencePositions.

Commit migrated from https://github.com/dotnet/corefx/commit/0c264c6d61cd6d18fbe7a760606f0e8548ff334d