tests: Improve diagnostics for malloc-failure tests

Some of these appear to hang, but making this diagnostic appear in the
log reveals the truth: it's just very slow.

Signed-off-by: Simon McVittie <smcv@collabora.com>
12 jobs for malloc-fail-diags in 8 minutes and 36 seconds (queued for 1 minute and 36 seconds)
Status Job ID Name Coverage
  Build
manual #395705
allowed to fail manual
build:buster
passed #395699
build:cmake

00:01:38

passed #395696
build:debug

00:07:58

manual #395701
allowed to fail manual
build:i686-w64-mingw32-cmake
passed #395700
build:i686-w64-mingw32-debug

00:03:57

manual #395704
allowed to fail manual
build:jessie
manual #395698
allowed to fail manual
build:legacy
passed #395695
build:production

00:04:46

manual #395697
allowed to fail manual
build:reduced
passed #395702
build:x86_64-w64-mingw32

00:03:38

passed #395703
build:x86_64-w64-mingw32-cmake-debug

00:04:52

manual #395706
allowed to fail manual
build:xenial