tests: Mark refcount/properties2 test as slow

It often takes a bit more than 30s to run on my local machine.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
8 jobs for 786-gio-test-leaks in 16 minutes and 17 seconds (queued for 3 minutes and 12 seconds)
Status Job ID Name Coverage
  Build
passed #58415
cross-android_api21_arm64

00:01:32

passed #58416
cross-android_api28_arm64

00:01:29

passed #58417
cross-mingw64

00:02:02

passed #58426
fedora-x86_64

00:04:04

passed #58418
win32
msys2-mingw32

00:05:56

passed #58419
win32
vs2017-x64

00:05:46

failed #58414
fedora-x86_64

00:05:53

 
  Coverage
passed #58420
coverage

00:01:12

80.1%