feat(coverage): add allowExternal option

[vojvodics]

Description

Implements allowExternal option removed in v0.34.0

Resolves #3867

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[AriPerkkio]

I'm not sure if setting allowExternal: true for all coverage tests is good approach. But let's do that for now and I'll see if better way to test this is possible.

[vojvodics]

@AriPerkkio I had the idea to run the tests via cli argument (e.g. "test:v8": "node ./testing.mjs --provider v8 && node ./testing.mjs --provider v8 --allowExternal") But.. I didn't manage to make it work - not sure if it's possible to have conditional snapshots (that could automatically be updated, not by matching inline snapshot)?
What I basically want is if allowExternal is passed to check if it matches one snapshot, and if not if it matches the original snapshot (everything currently points me to having 2 separate folders is the solution to having separate tests for both options).

[AriPerkkio]

Maybe we should have a separate testing.mjs script for testing different coverage options like allowExternal. It could be a single script in package.json.

Something like:

// vitest/test/coverage-test/testing-options.mjs

import { startVitest } from 'vitest/node'

const testCases = [
  {
    testConfig: {
      include: ['test/allow-external.test.ts'],
      coverage: { allowExternal: true, include: ['**/src/**', '**/test-utils/fixtures/**'] },
    },
    assertionConfig: {
      include: ['coverage-report-tests/allow-external.test.ts'],
    },
  },
]

for (const provider of ['v8', 'istanbul']) {
  for (const { testConfig, assertionConfig } of testCases) {
    // Run test case
    await startVitest('test', ['tests/'], {
      name: `With settings: ${JSON.stringify(testConfig)}`,
      ...testConfig,
      coverage: {
        enabled: true,
        provider,
        ...testConfig.coverage,
      },
    })

    // Check generated coverage report
    await startVitest('test', ['coverage-report-tests'], {
      ...assertionConfig,
    })
  }
}

And then we could leave the regular testing.mjs without allowExternal.

[AriPerkkio]

I've refactored the testing setup of test/coverage-test. Now both true|false values of allowExternal are tested.

Thanks @vojvodics!