libsel4test: fix compilation on GCC 9

Each 'struct testcase' is aligned to 64 bytes. Use the testcase_t type
instead of 'struct testcase' declaring the array to make the compiler
realise this and avoid -Waddress-of-packed-member.

libsel4test/include/sel4test/test.h

diff --git a/libsel4test/include/sel4test/test.h b/libsel4test/include/sel4test/test.h
index 459d044..60bab8f 100644
--- a/libsel4test/include/sel4test/test.h
+++ b/libsel4test/include/sel4test/test.h
@@ -155,8 +155,8 @@
 extern struct test_type __stop__test_type[];
 
 /* Definitions so that we can find the test cases */
-extern struct testcase __start__test_case[];
-extern struct testcase __stop__test_case[];
+extern testcase_t __start__test_case[];
+extern testcase_t __stop__test_case[];
 
 static inline int test_type_comparator(const void *a, const void *b)
 {