[release/1.4.7] eth/downloader: fix occasional fast sync critical section test fails

(cherry picked from commit 7832890)

Author: karalabe

eth/downloader/downloader_test.go

@@ -1824,13 +1824,15 @@ func testFastCriticalRestarts(t *testing.T, protocol int) {
 	for i := 0; i < fsPivotInterval; i++ {
 		tester.peerMissingStates["peer"][headers[hashes[fsMinFullBlocks+i]].Root] = true
 	}
+	tester.downloader.dropPeer = func(id string) {} // We reuse the same "faulty" peer throughout the test
+
 	// Synchronise with the peer a few times and make sure they fail until the retry limit
 	for i := 0; i < fsCriticalTrials; i++ {
 		// Attempt a sync and ensure it fails properly
 		if err := tester.sync("peer", nil, FastSync); err == nil {
 			t.Fatalf("failing fast sync succeeded: %v", err)
 		}
-		time.Sleep(500 * time.Millisecond) // Make sure no in-flight requests remain
+		time.Sleep(100 * time.Millisecond) // Make sure no in-flight requests remain
 
 		// If it's the first failure, pivot should be locked => reenable all others to detect pivot changes
 		if i == 0 {