feat: remove testFiles reference

[estrada9166]

• Closes UNIFY-1278

User facing changelog

Removes reference to testFiles and throw an error if it's being used on the root level of the config file, explaining that it has been removed for specPattern and where to set it!

Additional details

How has the user experience changed?

PR Tasks

• Have tests been added/updated?
• Has the original issue (or this PR, if no issue exists) been tagged with a release in ZenHub? (user-facing changes only)
• Has a PR for user-facing changes been opened in cypress-documentation?
• Have API changes been updated in the type definitions?
• Have new configuration options been added to the cypress.schema.json?

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[cypress]

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Test summary

17787 • 0 • 217 • 0 • 1

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Run details

Project	cypress
Status	Passed
Commit	c68c06b
Started	Mar 10, 2022 7:56 PM
Ended	Mar 10, 2022 8:08 PM
Duration	12:07 💡
OS	Linux Debian - 10.10
Browser	Multiple

View run in Cypress Dashboard ➡️

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Flakiness

	cypress/e2e/commands/xhr.cy.js		1
1	... > no status when request isnt forced 404

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[lmiller1990]

Nice