core/state, tests: fix memory leak via fastcache (#28387)

This change fixes a memory leak, when running either state-tests or blockchain-tests, we allocate a `1MB` fastcache during snapshot generation. `fastcache` is a bit special, and requires a `Reset()` (it has it's own memory allocator). 

The `1MB` was hidden [here](https://github.com/ethereum/go-ethereum/blob/master/tests/state_test_util.go#L333) and [here](https://github.com/ethereum/go-ethereum/blob/master/tests/block_test_util.go#L146) respectively.

core/state/snapshot/disklayer.go

@@ -45,6 +45,16 @@ type diskLayer struct {
 	lock sync.RWMutex
 }
 
+// Release releases underlying resources; specifically the fastcache requires
+// Reset() in order to not leak memory.
+// OBS: It does not invoke Close on the diskdb
+func (dl *diskLayer) Release() error {
+	if dl.cache != nil {
+		dl.cache.Reset()
+	}
+	return nil
+}
+
 // Root returns  root hash for which this snapshot was made.
 func (dl *diskLayer) Root() common.Hash {
 	return dl.root

core/state/snapshot/snapshot.go

@@ -656,6 +656,13 @@ func diffToDisk(bottom *diffLayer) *diskLayer {
 	return res
 }
 
+// Release releases resources
+func (t *Tree) Release() {
+	if dl := t.disklayer(); dl != nil {
+		dl.Release()
+	}
+}
+
 // Journal commits an entire diff hierarchy to disk into a single journal entry.
 // This is meant to be used during shutdown to persist the snapshot without
 // flattening everything down (bad for reorgs).