051e9f128e
The test asserting process.listenerCount("beforeExit") lived in the
parallel wait-flush.test.tsx, conflicting with the branch's rule that
process-global-state tests live in *.sequential.test.* files. Move it into
leak.sequential.test.tsx (already the home for process exit/SIGINT listener-
count assertions) and extend that file's header comment. #26 (item 2).