Error when running all snapshots

Hi, thanks for the amazing plugin!!!

There is a possible bug, as when I run all tests in the cypress browser I receive following error:

CypressError: cy.task('cypress-plugin-snapshot:matchText') failed with the following error:

> Error: Cannot load snapshot file. File "__snapshots__/__all.snap does not contain valid JSON or javascript

[bahmutov]

So the workaround I can offer for now is to fix Cypress.spec object by looking at the __filename in each spec file. Here is the code to place into cypress/support/index.js. It is a utility function that forms Cypress.spec from passed filename

// cypress/support/index.js
export const fixCypressSpec = filename => () => {
  const path = require('path')
  const relative = filename.substr(1) // removes leading "/"
  const projectRoot = Cypress.config('projectRoot')
  const absolute = path.join(projectRoot, relative)
  Cypress.spec = {
    absolute,
    name: path.basename(filename),
    relative
  }
}

From each spec file (that wants to use snapshots)

import { fixCypressSpec } from '../support'

beforeEach(fixCypressSpec(__filename))

I have been playing with this in https://github.com/bahmutov/objection-example using this approach to run all specs with snapshots

[sgnl]

IMO this issue should be mentioned on the README as it’s a huge blocker for me, I only found out after implementing and writing tests (using .only while writing tests) to find the error in the GUI test runner after I ran all my tests which made all my work previously useless.

Please consider adding an eye-catching warning or ‘caveat’ section which mentions this issue so that other users of this package will not share the same experience I had or at the very least, know of this important bug.

Hope Cypress gets the fix in soon, :fingers

(added a PR as a suggestion for a new section in the README file: https://github.com/meinaart/cypress-plugin-snapshots/pull/37)