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.

build/ci.go

@@ -285,6 +285,7 @@ func doTest(cmdline []string) {
 		coverage = flag.Bool("coverage", false, "Whether to record code coverage")
 		verbose  = flag.Bool("v", false, "Whether to log verbosely")
 		race     = flag.Bool("race", false, "Execute the race detector")
+		short     = flag.Bool("short", false, "Pass the 'short'-flag to go test")
 		cachedir = flag.String("cachedir", "./build/cache", "directory for caching downloads")
 	)
 	flag.CommandLine.Parse(cmdline)
@@ -318,6 +319,9 @@ func doTest(cmdline []string) {
 	if *race {
 		gotest.Args = append(gotest.Args, "-race")
 	}
+	if *short {
+		gotest.Args = append(gotest.Args, "-short")
+	}
 
 	packages := []string{"./..."}
 	if len(flag.CommandLine.Args()) > 0 {