execution/commitment: fix flaky BranchCache unwind tests

execution/commitment/branch_cache_test.go

@@ -194,11 +194,20 @@ func TestBranchCache_Stats(t *testing.T) {
 	require.True(t, strings.HasPrefix(s, "branch-cache "))
 }
 
+// newUnwindTestCache sizes the tail large enough that LRU eviction can't
+// interfere with the txN/epoch invalidation these tests exercise. The tail is
+// sharded (branchCacheTailShards); under a tiny capacity most shards hold a
+// single entry, so two keys hashing into the same shard evict each other
+// depending on the per-process hash seed.
+func newUnwindTestCache() *BranchCache {
+	return NewBranchCache(DefaultBranchCacheTailCapacity)
+}
+
 // TestBranchCache_Unwind_DropsStaleAboveFloorLazily verifies Unwind drops
 // (lazily, on the next Get) every superseded-epoch entry whose txN is at or
 // above the unwind floor, while entries below the floor survive untouched.
 func TestBranchCache_Unwind_DropsStaleAboveFloorLazily(t *testing.T) {
-	c := NewBranchCache(100)
+	c := newUnwindTestCache()
 
 	rootKey := []byte{0x00}
 	tailKeyKeep := []byte{0xa0, 0xb0}
@@ -243,7 +252,7 @@ func TestBranchCache_Unwind_AcrossAllTiers(t *testing.T) {
 // entry stamped exactly at the unwind floor belongs to a rolled-back block and
 // drops; an entry one txN below the floor survives.
 func TestBranchCache_Unwind_FloorBoundary(t *testing.T) {
-	c := NewBranchCache(100)
+	c := newUnwindTestCache()
 
 	belowKey := []byte{0xa0, 0xb0}
 	atKey := []byte{0xa0, 0xb1}