selftests/bpf: libarena: Replace leftover st_ prefix with test_

The st_ (selftests_) prefix is confusing and has been replaced
with the more descriptive test_. However, last patch did not
properly move all files to the new prefix. Rename the existing
files to complete the move.

Acked-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://lore.kernel.org/bpf/20260706181730.21731-2-emil@etsalapatis.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

tools/testing/selftests/bpf/libarena/selftests/test_asan_buddy.bpf.c

@@ -12,7 +12,7 @@ extern struct buddy __arena buddy;
 
 #ifdef BPF_ARENA_ASAN
 
-#include "st_asan_common.h"
+#include "test_asan_common.h"
 
 static __always_inline int asan_test_buddy_oob_single(size_t alloc_size)
 {