Update CHANGELOG.md

Author: keveleigh

org.mixedrealitytoolkit.core/CHANGELOG.md

@@ -7,6 +7,11 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 ### Changed
 
 * Updated code style in `HandsSubsystemDescriptor`, `MRTKSubsystemDescriptor`, `DictationSubsystemDescriptor`, and `XRSubsystemHelpers`. [PR #1109](https://github.com/MixedRealityToolkit/MixedRealityToolkit-Unity/pull/1109)
+* `AssemblyExtensions.GetLoadableTypes` now throws `ArgumentNullException` when called on a null assembly. [PR #1122](https://github.com/MixedRealityToolkit/MixedRealityToolkit-Unity/pull/1122)
+
+### Added
+
+* Added edit mode tests for `AssemblyExtensions`, `SystemType`, and `SerializableDictionary`. [PR #1122](https://github.com/MixedRealityToolkit/MixedRealityToolkit-Unity/pull/1122)
 
 ### Fixed