Fix intermittent abort when running tests on macOS

The problem was again that tests weren't cleaning up after themselves
properly, and were then breaking subsequent tests: this time, they were
correctly calling `NocStorage::clear_noc`, but that was failing to clear
`router_id_conversion_table`.

Since `unordered_map::emplace` is used to add entries to that table,
writing to an entry that's already been filled doesn't do anything, and
so subsequent reads wil retrieve the data from previous tests: the
retrieved IDs are then used as indices, causing out-of-bounds array
accesses and an eventual crash.

Author: Liamolucko

vpr/src/noc/noc_storage.cpp

@@ -256,6 +256,7 @@ void NocStorage::clear_noc() {
     link_storage.clear();
     router_outgoing_links_list.clear();
     router_incoming_links_list.clear();
+    router_id_conversion_table.clear();
     grid_location_to_router_id.clear();
 
     built_noc = false;