Fix segfault while running tests on macOS

The tests run in a different order on macOS than Linux, which happens to
result in this segfault.

The problem is that `test_compressed_grid` leaves some stuff behind in
`mutable_device().logical_block_types`, which have `pb_type` set to
`nullptr`. This then breaks `test_connection_router`'s call to
`XmlReadArch`, since it assumes that all of the logical block types are
ones that it created, all of which either have `pb_type` set or are
empty.

Making `test_compressed_grid` clear `logical_block_types` when it's done
fixes that.

Author: Liamolucko

vpr/test/test_compressed_grid.cpp

@@ -330,6 +330,8 @@ TEST_CASE("test_compressed_grid", "[vpr_compressed_grid]") {
         REQUIRE(grid_loc == t_physical_tile_loc{98, 98, 0});
     }
 
+    logical_block_types.clear();
+
 }
 
 } // namespace