Harden test for no new acquires after transaction abort

If a transaction is doomed we want to gracefully shutdown our zoo of
worker processes. As explained in the referenced commit we do this by
stopping the main process from handing out new work and ignoring the
replies it gets from the workers, so that they eventually run out of
work.

We tested this previously by checking if a rred worker was given work
items at all, but depending on how lucky the stars of the machine
working on this are the worker would have already gotten work before the
transaction was aborted – so we tried this 25 times a row (f35601e).
No machine can be this lucky, right?

Turns out the autopkgtest armhf machine is very lucky.

I feel a bit sorry for feeding grep such a long "line" to work with, but
it seems to work out. Porterbox amdahl (who is considerably less lucky;
had to turn down to 1 try to get it to fail sometimes) is now happily
running the test in an endless loop.

Of course, I could have broken the test now, but its still a rather
generic grep (in some ways more generic even) and the main part of the
testcase – the update process finishes and fails – is untouched.

References: 38f8704
Closes: #984966

test/integration/test-pdiff-usage

@@ -254,16 +254,10 @@ SHA256-Download:
 	cp Packages-future aptarchive/Packages
 	rm -f rootdir/var/lib/apt/lists/*_Contents-*
 	webserverconfig 'aptwebserver::overwrite::.*Contents-.*::filename' '/hacked-i386.gz'
-	# This should work in at least 4% of the cases...
-	for i in $(seq 25); do
-		testfailure apt update "$@"
-		if ! grep -q 'rred:600' rootdir/tmp/testfailure.output; then
-			break
-		fi
-	done
-	webserverconfig 'aptwebserver::overwrite::.*Contents-.*::filename' '/Contents-i386.gz'
+	testfailure apt update "$@"
 	cp rootdir/tmp/testfailure.output patchdownload.output
-	testfailure grep 'rred:600' patchdownload.output
+	webserverconfig 'aptwebserver::overwrite::.*Contents-.*::filename' '/Contents-i386.gz'
+	testfailure grep -q -z 'AbortTransaction.* [a-zA-Z0-9+-]\+:600%20' patchdownload.output
 	testnopackage newstuff futurestuff
 	testsuccessequal "$(cat "${PKGFILE}")
 " aptcache show apt oldstuff