Fix intermittently failing test (#8626)

`app::background::tasks::inventory_collection::test::test_basic` fails
intermittently, more often when run as part of a large batch. The
problem seems to be the race between manual inventory collection and
Nexus' own background task. This PR accounts for the race (but does not
eliminate it) by allowing for one extra Nexus-initiated collection. More
than one seems theoretically possible but extremely unlikely in this
test.

---------

Co-authored-by: John Gallagher <john@oxidecomputer.com>

Author: plotnick

nexus/src/app/background/tasks/inventory_collection.rs

@@ -355,8 +355,14 @@ mod test {
             assert!(num_collections > 0);
 
             // Regardless of the activation source, we should have at
-            // most `nkeep + 1` collections.
-            assert!(num_collections <= nkeep + 1);
+            // most `nkeep + 2` collections: at most `nkeep + 1` from
+            // our collection, and at most one more if we lose the race
+            // with Nexus and it happens to insert another collection
+            // after we pruned down to `nkeep`.
+            assert!(
+                num_collections <= nkeep + 2,
+                "expected {num_collections} <= {nkeep} + 2"
+            );
 
             // Filter down to just the collections we activated. (This could be
             // empty if Nexus shoved several collections in!)