miner: fix data race during shutdown (#23435)

This fixes a data race on worker.current by moving the call to StopPrefetcher
into the main loop.

The commit also contains fixes for two other races in unit tests of unrelated packages.

eth/gasprice/gasprice_test.go

@@ -151,7 +151,7 @@ func newTestBackend(t *testing.T, londonBlock *big.Int, pending bool) *testBacke
 	// Construct testing chain
 	diskdb := rawdb.NewMemoryDatabase()
 	gspec.Commit(diskdb)
-	chain, err := core.NewBlockChain(diskdb, nil, gspec.Config, engine, vm.Config{}, nil, nil)
+	chain, err := core.NewBlockChain(diskdb, &core.CacheConfig{TrieCleanNoPrefetch: true}, gspec.Config, engine, vm.Config{}, nil, nil)
 	if err != nil {
 		t.Fatalf("Failed to create local chain, %v", err)
 	}

eth/handler_eth_test.go

@@ -486,7 +486,6 @@ func TestCheckpointChallenge(t *testing.T) {
 }
 
 func testCheckpointChallenge(t *testing.T, syncmode downloader.SyncMode, checkpoint bool, timeout bool, empty bool, match bool, drop bool) {
-	t.Parallel()
 
 	// Reduce the checkpoint handshake challenge timeout
 	defer func(old time.Duration) { syncChallengeTimeout = old }(syncChallengeTimeout)