Add unit testcase for RefreshAndStoreToken function

[Kshitij-Katiyar]

          @Kshitij-Katiyar This is where much of the functionality was added in this PR. I'm thinking we can test this function and `CheckUserConnected` directly. What do you think?

We basically want to emulate this in a test since this is the code that is applied throughout the PR:

	if !c.tokenHelpers.CheckUserConnected(c.mattermostUserID) {
		c.Logger.Warnf(LogUserInactive, c.mattermostUserID)
		return nil, errors.New(ErrorUserInactive)
	}
	err := someCallToAPI // this can be whatever we want in the test
	if err != nil {
		c.tokenHelpers.DisconnectUserFromStoreIfNecessary(err, c.mattermostUserID)

We can use a "real" KV store in the test with one of the following strategies:

• https://gist.github.com/mickmister/f2ac1c99c63291692b18fdae715338af
• Adopt memorystore for tests mattermost-plugin-github#745

Originally posted by @mickmister in #256 (comment)