tests: add in-tree testing tool to list allocations within mark

This would be nice to have more generally useful via filters in the
allocations view. But for now, just make something simple work.

src/tests/meson.build

@@ -81,6 +81,12 @@ allocs_by_size = executable('allocs-by-size',
   dependencies: test_deps,
 )
 
+allocs_within_mark = executable('allocs-within-mark',
+  ['allocs-within-mark.c'],
+        c_args: test_cflags,
+  dependencies: test_deps,
+)
+
 cross_thread_frees = executable('cross-thread-frees',
   ['cross-thread-frees.c'],
         c_args: test_cflags,