Added example for running specific scenario via cli

[taylorflatt]

Why?

On a number of occasions I have found colleagues wanting to run a specific scenario of a given test script. Early on I saw folks would just comment things out to get to the behavior they wanted or would start nesting scripts and fanning tests out.

What?

• Added an example to the scenario section outlining how to effectively use an environment variable to run a specific scenario among a group.
• Tested example in all versions that it works as expected.

Checklist

• I have used a meaningful title for the PR.
• I have described the changes I've made in the "What?" section above.
• I have performed a self-review of my changes.
• I have run the npm start command locally and verified that the changes look good.

• I have made my changes in the docs/sources/next folder of the documentation.
• I have reflected my changes in the docs/sources/v{most_recent_release} folder of the documentation.
• I have reflected my changes in the relevant folders of the two previous k6 versions of the documentation (if still applicable to previous versions).

Related PR(s)/Issue(s)

• Option to toggle the scenario run k6#2780

[taylorflatt]

Any thoughts @heitortsergent?

[heitortsergent]

Hi @taylorflatt! Sorry for the delay here 🙏 .

I checked with the team and they agreed that this is a good suggestion! I just want to review some of the wording, I should be able to do that tomorrow and then we can get this merged. And thanks for opening the PR! 🙇

[taylorflatt]

Excellent! No problemo.

I installed vale locally and it doesn't seem like it is picking up any blockers when I point to either the .vale configuration in k6-docs or the writers-toolkit. The error in the action for prose linting is odd and if a real error on my side a bit confusing:

  E100 [loadINI/homedir] Runtime error
  
  $HOME is not defined

Do you have any direction on this?

[heitortsergent]

@taylorflatt I left some suggestions on the wording, let me know what you think. 🙏

[heitortsergent]

Suggested change

	Utilizing an [environment variable](https://grafana.com/docs/k6/<K6_VERSION>/using-k6/environment-variables/#environment-variables) and a bit of code, you can run a specific scenario via command line as opposed to running all configured scenarios. By default, [k6 runs all scenarios listed in a file](https://grafana.com/docs/k6/<K6_VERSION>/using-k6/scenarios/#scenarios).
	k6 runs all [scenarios](https://grafana.com/docs/k6/<K6_VERSION>/using-k6/scenarios/#scenarios) listed in a test script by default. But, with some small code changes and using [environment variables](https://grafana.com/docs/k6/<K6_VERSION>/using-k6/environment-variables/#environment-variables), you can tell k6 to only run a specific scenario via the command-line.

I think if we change the order of the sentences here, it might be a little bit easier to read.

[heitortsergent]

Suggested change

	The following provides an example of how to add the ability to run a specific scenario among multiple configured scenarios:
	The following example shows a test script that uses a `SCENARIO` environment variable, if it exists, to choose which scenario to execute:

[heitortsergent]

Suggested change

	// if a scenario is supplied via cli env, then run that scenario. Otherwise, run like normal
	// if a scenario is passed via a CLI env variable, then run that scenario. Otherwise, run

[heitortsergent]

Suggested change

	Alternatively, slightly modifying this approach, it can instead be used as a skip feature. Reading from the custom environment variable allows calling a specific scenario via command line:
	Then from the command line, you could run the test script and only execute the `my_web_test` scenario by running:

I think we can just give an instruction for the user as to how they can execute this from the command line. If we want to mention using this as a skip feature, I think we should do it after the command line examples, or in the first section.

[heitortsergent]

Oh, and don't worry about the Vale check in the PR. I need to fix it so it's consistent with the writers-toolkit one we use in other repositories.