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 make-4.3 in 7 minutes and 44 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Test
manual build:arch #37566
allowed to fail manual
passed build:bionic #37565

00:07:43

passed build:buster #37564

00:06:47

passed build:stretch #37563

00:07:33