tests: Fix a -Wbad-function-cast warning in the atomic tests

Assigning the gpointer return value from g_atomic_pointer_get() to a
gsize variable was not making -Wbad-function-cast happy. Assign to an
intermediate gpointer variable and then cast that instead.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

GNOME/glib#1475
8 jobs for 1475-atomic-casts in 20 minutes and 6 seconds (queued for 11 seconds)
Status Job ID Name Coverage
  Build
passed #77700
cross-android_api21_arm64

00:01:40

passed #77701
cross-android_api28_arm64

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passed #77702
cross-mingw64

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passed #77699
fedora-x86_64

00:04:26

manual #77705
freebsd-11 allowed to fail manual
freebsd-11-x86_64
passed #77703
win32
msys2-mingw32

00:08:34

passed #77704
win32
vs2017-x64

00:05:41

 
  Coverage
passed #77706
coverage

00:01:36

73.3%