tests: Cope with newer versions of GNU make

The file descriptor pointing to the jobserver is not passed through
the test script, so passing through a MAKEFLAGS environment variable
that refers to it is just going to confuse us.

Signed-off-by: Simon McVittie <smcv@collabora.com>
4 jobs for master in 8 minutes and 7 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
manual #37766
allowed to fail manual
build:arch
passed #37765
build:bionic

00:08:07

passed #37764
build:buster

00:07:39

passed #37763
build:stretch

00:07:47