test: use slackcontext.MockContext(...) instead of context.Background()#28

mwbrooks merged 5 commits intomainslackapi/slack-cli:mainfrom

mbrooks-slackcontext-tabletestcommand-expectedasserts-backgroundslackapi/slack-cli:mbrooks-slackcontext-tabletestcommand-expectedasserts-backgroundCopy head branch name to clipboard