tapgarden: increase unit test default timeout from 10s to 30s

The `unit-race` CI job frequently flakes due to test timeouts.
Raising the default timeout to 30s should help reduce these flakes
without noticeably slowing down the unit tests.

Author: ffranr

tapgarden/planter_test.go

@@ -46,7 +46,7 @@ import (
 // Default to a large interval so the planter never actually ticks and only
 // rely on our manual ticks.
 var (
-	defaultTimeout    = time.Second * 10
+	defaultTimeout    = time.Second * 30
 	noCaretakerStates = fn.NewSet(
 		tapgarden.BatchStatePending,
 		tapgarden.BatchStateSeedlingCancelled,