Remove flakiness in LongTasksAPI test (#52974)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/52974

Changelog: [internal]

The test for LongTasks is flaky on Github and I made a change to figure out why (D79366370 / https://github.com/facebook/react-native/pull/52948). It seems that the long task happens before the artificial task we're using for testing, so we can update the test to filter those out.

Reviewed By: cortinico

Differential Revision: D79441987

fbshipit-source-id: 99296d704cfec2e61ca29d06878df171231f4e78