Update TestContext.Properties return type to IDictionary<string, object>

[Evangelink]

Summary

Background and Motivation

Currently, the TestContext.Properties property returns IDictionary for compatibility reason. This is forcing users to rely on old-aged API where they could be using API coming from Dictionary<string, object>.

Proposed Feature

Replace IDictionary with IDictionary<string, object> .

AB#1707845

[Youssef1313]

@Evangelink I'm re-opening as the PR that closed this issue was reverted