tanzu test plugin <plugin-name> should throw proper error when the test plugin is not installed

[anujc25]

Bug description

Currently, when we run tanzu test plugin <plugin-name> it runs the test plugin binary of a given plugin.
However, if the test plugin is not installed for the given plugin it doesn't mention about missing test plugin but just shows help text as below:

$ tanzu test plugin cluster
Plugin tests

Usage:
  tanzu test plugin [command]

Available Commands:
  test        Test the CLI

Flags:
  -h, --help   help for plugin

Use "tanzu test plugin [command] --help" for more information about a command.

Showing a proper error message instead of showing helptext would make lot of sense in this case.

Affected product area (please put an X in all that apply)

• ( ) APIs
• ( ) Addons
• (x) CLI
• ( ) Docs
• ( ) IAM
• ( ) Installation
• ( ) Plugin
• ( ) Security
• ( ) Test and Release
• ( ) User Experience
• ( ) Developer Experience

Expected behavior

• Show proper error message if the test plugin is not installed

Steps to reproduce the bug

Version (include the SHA if the version is not obvious)

Environment where the bug was observed (cloud, OS, etc)

Relevant Debug Output (Logs, manifests, etc)