tests: posix: xsi_realtime: Set enough file descriptors

Earlier fix in commit bfd033d
caused the Posix xsi_realtime test to fail. Set minimum amount
of file descriptors in the test to pass the build assert.

Signed-off-by: Jukka Rissanen <[email protected]>

Author: jukkar

tests/posix/xsi_realtime/prj.conf

@@ -13,3 +13,6 @@ CONFIG_ZTEST_STACK_SIZE=2048
 CONFIG_DYNAMIC_THREAD=y
 CONFIG_THREAD_STACK_INFO=y
 CONFIG_DYNAMIC_THREAD_POOL_SIZE=6
+
+CONFIG_ZVFS_OPEN_IGNORE_MIN=y
+CONFIG_ZVFS_OPEN_MAX=5