feat(test-runner-commands): make snapshot file path configurable

[lovenm]

What I did

1. Added configuration option to control where snapshot files are saved.

I was not totally sure if the configuration of the plugin should happen somewhere other than TestRunnerConfig, but could not figure a better way to configure it since the plugin seems to be added by default when the runner is started. If I understood correctly, if a user would add the snapshotPlugin(...) in their own configuration file, that would not override the default one, right?

I've not added changesets or updated the docs yet, but can do that if the code changes otherwise look ok.

[changeset-bot[bot]]

⚠️ No Changeset found

Latest commit: 9648e62c65d2076bb08e1cae3949f39d5dc27639

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[ca-d]

@koddsson @bashmish would you please comment on whether the behaviour described in #1521 is intended or, if not, what (apart from a changeset) would be needed to accept this pull request? Is the missing changeset the only reason this issue and pull request don't see any activity?

Recap of the situation: We currently have to commit an empty dist directory with only a __snapshots__ subdirectory to our repository just because we're no longer able to configure the snapshot directory ever since moving away from karma.

[lovenm]

Honestly it's been such a long time since I submitted this, that I'm a bit out of the loop regarding the codebase and have no idea how relevant the changes are anymore. The conflicts don't look too outlandish I suppose.

I've also since left the project where we were struggling with this issue, but can maybe give this a look at if this is deemed relevant in some way.