Clean up query_stats_store_test.cpp after all read errors unit tests are added.

Some things to clean up:

• Add a QueryStatsErrorsTest : QueryStatsStoreTest fixture holding the flag guard as a member so unittest::ServerParameterGuard errorsEnabled("featureFlagQueryStatsErrors", true) isn't repeated
• If SERVER-131927 changes makeFindKeyFromQuery to return unique_ptr<key> instead of const key, then the lambda on 344-347 can be removed (currently builds the key twice - once to hash and once to pass into writeQueryStats. That was a workaround for makeFindKeyFromQuery returning unique_ptr<const Key
• Add assertErrorState(entry, {execCountErrored, numRecentErrors, code}) to remove duplicated execCountErrored / recentErrors.size() / recentErrors.hasKey() checks