rust: project1 error test cases are passed with incorrect implementation

[miliya612]

Hello everyone, thank you for great course.

I was proceeding the project1, and I found that below tests are passed without specifying clap's required flag, which validates arguments for subcommand

• cli_invalid_get
• cli_invalid_set
• cli_invalid_rm

It would help reader if we could add validation for error outputs of these tests.
That requires dependencies to clap's message, so might be debatable.