tests: Mark two more tests as slow

These keep on taking just longer than 30s on my local machine when run
in parallel with the rest of the tests (i.e. with `ninja test`). Testing
them individually, they do terminate correctly.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
9 jobs for <span class="ref-name">slow-tests</span> in 26 minutes and 1 second (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #108032
cross-android_api21_arm64

00:01:35

passed #108033
cross-android_api28_arm64

00:01:35

passed #108034
cross-mingw64

00:02:11

passed #108031
fedora-autotools-x86_64

00:05:47

passed #108030
fedora-x86_64

00:04:31

manual #108037
freebsd-11 allowed to fail manual
freebsd-11-x86_64
passed #108035
win32
msys2-mingw32

00:08:35

passed #108036
win32
vs2017-x64

00:05:37

 
  Coverage
passed #108038
coverage

00:01:45

74.1%