Use relaxed JavascriptEncoder in S.T.JSON specific tests to improve coverage. (dotnet/corefx#39560)
* Use relaxed JavascriptEncoder in S.T.JSON specific tests to improve
coverage.
* Add control characters as well as part of the test data.
* Add JsonElement.WriteTo test and make sure all Utf8JsonWriter in tests
are disposed.
* Fix spacing/formatting.
* Fix call to AssertEquals since it moved to JsonTestHelper.
Commit migrated from https://github.com/dotnet/corefx/commit/
74d26be704117139e9ef081c9e287740fde5d002