libsel4test: Add device_frame to env

We add this struct member to fix the functionality of a test. More
specifically, the test where we assign a device frame to a thread's IPC
buffer.

libsel4test/include/sel4test/test.h

diff --git a/libsel4test/include/sel4test/test.h b/libsel4test/include/sel4test/test.h
index d4122d3..459d044 100644
--- a/libsel4test/include/sel4test/test.h
+++ b/libsel4test/include/sel4test/test.h
@@ -70,6 +70,7 @@
 #endif
     seL4_Word cores;
     seL4_CPtr domain;
+    seL4_CPtr device_frame;
 
     int priority;
     int cspace_size_bits;