tap tests on older branches fail if concurrency is used

Hi,

when using
$ cat ~/.proverc
-j9

some tests fail for me in 9.4 and 9.5. E.g. src/bin/script's tests
yields a lot of fun like:
$ (cd ~/build/postgres/9.5-assert/vpath/src/bin/scripts/ && make check)
...
# LOG: received immediate shutdown request
# WARNING: terminating connection because of crash of another server process
# DETAIL: The postmaster has commanded this server process to roll back the current transaction and exit, because another server process exited abnormally and possibly corrupted shared memory.
# HINT: In a moment you should be able to reconnect to the database and repeat your command.
...

it appears as if various tests are trampling over each other. If needed
I can provide detailed logs, but it appears to readily reproduce on
several machines...

See Michael, I'll provide the details and a reproducer ;)

Greetings,

Andres Freund