Most suites already did so via start_test_server(), but the pg_rewind,
pg_ctl and pg_controldata suites ran a postmaster or initdb with fsync
enabled.  This halves the pg_rewind suite's runtime on buildfarm member
tern.  It makes tern and that machine's other buildfarm members less
vulnerable to noise failures from postmaster startup overrunning the 60s
pg_ctl timeout.  Back-patch to 9.5, where pg_rewind was introduced.