add "testOptions.files" support

[FredKSchott]

Changes

• Needed by Better test runner for Snowpack apps #1036
• To run tests through Snowpack's build pipeline (instead of a secondary, custom Jest build pipeline) we'll need to install dependencies for test files just like we do source files.
• Currently, we recommend adding test files to the "exclude" config. This means they'll never be scanned.
• Instead, we want to identify a new type of config called "testOptions.files", which tells Snowpack "don't include these in the final build, but do scan them for dependencies when in test mode"

Testing

• TODO (will wait for first review to confirm direction)

Docs

• Added.

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[melissamcewen]

Looks great on the docs side

[gr2m]

Looks good to me 👍🏼

[drwpow]

Just my opinion: I’d put a link here stating that this pattern uses glob’s ignore option. For me, I’m always confused what I can/can’t do when it comes to glob patterns like this (is * recursive or not? do I need to start each with a !? etc.).

[FredKSchott]

ah agreed, but not sure if it makes sense to document the ins and out of globs each time we document something that supports globs. Maybe have some overall glob support documentation?