sim: Avoid -Werror=unused-variable in bufval unittests

Some compilers (e.g. gcc-7.5) throw a warning/error when a structured
binding variable is not used.

Change-Id: Ib5090508d135e4a72c39f18b316c76674c3bd8f3
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/57229
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/sim/bufval.test.cc

@@ -40,8 +40,7 @@ template <class First>
 bool
 pairFail(const std::pair<First, bool> &p)
 {
-    auto &[first, success] = p;
-    return !success;
+    return !p.second;
 }
 
 bool