make sampling_test more robust

The test inspects "nested-most" frames in malloc sampling samples. And
some compile options caused noopt to seen as call site of
malloc. I.e. because noopt is immediately after malloc.

To make things more robust we create local "copy" of noopt that is
marked as noinline and so immediately next instruction after malloc is
call to this local_noopt thingy, which makes test more robust since
malloc stack trace will see that, as we expect, AllocateAllocate is
what calls malloc.

src/tests/sampling_test.cc

@@ -67,7 +67,10 @@ static std::string NaiveShellQuote(std::string_view arg) {
 
 extern "C" ATTRIBUTE_NOINLINE
 void* AllocateAllocate() {
-  return noopt(malloc(10000));
+  auto local_noopt = [] (void* ptr) ATTRIBUTE_NOINLINE -> void* {
+    return noopt(ptr);
+  };
+  return local_noopt(malloc(10000));
 }
 
 #ifndef PPROF_PATH