[libunwind] undef NDEBUG for assert.h in tests.

This makes sure the assertions also get verified in optimized builds.
This matches what is already done in bad_unwind_info.pass.cpp.

Reviewed By: #libunwind, MaskRay

Differential Revision: https://reviews.llvm.org/D131210

NOKEYCHECK=True
GitOrigin-RevId: fc6a6ee507ec2df0df7d34ba19feccb776297e4c
diff --git a/test/signal_unwind.pass.cpp b/test/signal_unwind.pass.cpp
index 2ff50ab..e6a53ab 100644
--- a/test/signal_unwind.pass.cpp
+++ b/test/signal_unwind.pass.cpp
@@ -13,6 +13,7 @@
 // TODO: Figure out why this fails with Memory Sanitizer.
 // XFAIL: msan
 
+#undef NDEBUG
 #include <assert.h>
 #include <dlfcn.h>
 #include <signal.h>