Do not prevent Extensions.FileProviders tests from running in browser (#56257)

Do not prevent Extensions.FileProviders tests from running in browser (#56257)

* Do not prevent Extensions.FileProviders tests from running in CI

* Remove SkipOnPlatform

* Do not throw PNSE for PhysicalFileProvider in browser, fallback to use polling instead

* Use SkipOnPlatform only on tests that use FileSystemWatcher

* Set UsePollingFileWatcher and UseActivePolling for browser to avoid ArgumentNullException in PhysicalFilesWatcher

* Fix Moq CI failures

* Use Assert.Same instead of Assert.Equals

* Disable tests for IChangeToken equality between multiple Watch() calls in Browser