samples: net: socketpair: ensure integration platform is on allow list

Ensure that qemu_x86 (the integration platform) is on the allow list
for `sample.net.sockets.socketpair.s32k148_evb`.

```
INFO: Error found: sample.net.sockets.socketpair.s32k148_evb on
qemu_x86/atom (Not in testsuite platform allow list but is one of the
integration platforms)
```

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>

Author: cfriedt

samples/net/sockets/socketpair/sample.yaml

@@ -8,10 +8,10 @@ common:
     - posix
   harness: net
   arch_exclude: posix
-  integration_platforms:
-    - qemu_x86
 tests:
   sample.net.sockets.socketpair:
+    integration_platforms:
+      - qemu_x86
     platform_exclude: s32k148_evb/s32k148
   sample.net.sockets.socketpair.s32k148_evb:
     extra_args: SHIELD=nxp_adtja1101