Fix variable duration race in lib/inventory tests (#49657)

lib/inventory/controller.go

@@ -266,13 +266,11 @@ func (c *Controller) RegisterControlStream(stream client.UpstreamInventoryContro
 	c.store.Insert(handle)
 
 	// Increment the concurrent connection counter that we use to calculate the
-	// variable instance heartbeat duration. It's done here synchronously rather
-	// than in handleControlStream for the sake of tests.
+	// variable instance heartbeat duration. To make the behavior more easily
+	// testable, we increment it here and we decrement it before closing the
+	// stream in handleControlStream.
 	c.instanceHBVariableDuration.Inc()
-	go func() {
-		defer c.instanceHBVariableDuration.Dec()
-		c.handleControlStream(handle)
-	}()
+	go c.handleControlStream(handle)
 }
 
 // GetControlStream gets a control stream for the given server ID if one exists (if multiple control streams
@@ -375,6 +373,7 @@ func (c *Controller) handleControlStream(handle *upstreamHandle) {
 			}
 		}
 
+		c.instanceHBVariableDuration.Dec()
 		for _, service := range handle.hello.Services {
 			c.serviceCounter.decrement(service)
 		}