Add test helpers for gating by feature

[acdlite]

Adds a new test helper, it.gate. You pass it the name of a feature flag, and if the flag is enabled, it asserts that the test passes. If the flag is off, it asserts that the test fails. Example:

it.gate('enableNewReconciler', 'test that only passes in new reconciler', () => {
  // ... test goes here
});

(test.gate is equivalent and makes perhaps makes more grammatical sense, but I'm using it.gate in these examples since we use it everywhere in our tests.)

You can test multiple flags by passing an array:

it.gate(
  ['enableNewReconciler', 'enableModernEventSystem'],
  'test that passes only in new reconciler with modern event system', () => {
    // ... test goes here
  }
);

If any of the flags are disabled, the test is expected to fail.

You can check if a flag is off by adding a ! to the front:

it.gate(
  '!enableNewReconciler',
  'test that passes only in the old reconciler', () => {
    // ... test goes here
  }
);

For anything more advanced, you can pass a function:

it.gate(
  flags => flags.warnAboutSpreadingKeyToJSX || !__DEV__,
  'should warn when keys are passed as part of props', () => {
    // Test passes when the flag is enabled, or in prod since
    // there are no warnings in prod
  }
);

I also added aliases for it.old and it.new, for testing the different reconciler forks:

it.old('test that passes only in the old reconciler', () => {
  // ... test goes here
});
it.new('test that passes only in the new reconciler', () => {
  // ... test goes here
});

Next steps

Longer term I think it would nice if these were implemented as pragmas instead:

// @gate enableNewReconciler
it('test that passes only in the new reconciler', () => {
  // ... test goes here
});

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[acdlite]

Update: Added this to PR description

Longer term I think it would nice if these were implemented as pragmas instead:

// @gate enableNewReconciler
it('test that passes only in the new reconciler', () => {
  // ... test goes here
});

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[sebmarkbage]

I kind of was hoping to switch to the test() form. Does this patch that too? I guess it’s just an alias for it?

[acdlite]

Yeah it is an alias

[acdlite]

I went ahead and opened an alternative PR for the pragma proposal: #18581

I prefer that one, but I'll keep this PR open for reference.

[acdlite]

Closed by #18581