Resolve "Implement interface for meltano `validator` plugins"

Merge Request Checklist

• Link to an issue
• Include the proposed fix or feature
• Include and update tests for the modified code
• Include a documentation change
• Add a CHANGELOG.md entry in the Unreleased section for any user-facing changes
• Mention or assign a maintainer

Updated Proposal

Commands that are also tests

Commands of any plugin whose names start with test are considered tests commands.

Modes of invocation

All available tests for all plugins

Runs all commands that are tests for all plugins.

meltano test --all`

All available tests for a single plugin

Runs all commands that are tests for a single plugin.

meltano test <plugin>

Named test

Runs a named test for a single plugin.

meltano test <plugin>:<test-name>

Named test (two or more plugins)

Runs named test for one or more plugins.

meltano test <plugin1>:<test-name1> <plugin2>:<test-name2>

Closes #2838 (closed)