test: test update for Go 1.12.5 and related changes

Update to Go 1.12.5 testing. Remove deprecated unused and gosimple
pacakges, and mask staticcheck 1006. Also, fix unconvert errors related
to unnecessary type conversions and following staticcheck errors:
- remove redundant return statements
- use for range instead of for select
- use time.Since instead of time.Now().Sub
- omit comparison to bool constant
- replace T.Fatal and T.Fatalf in tests with T.Error and T.Fatalf respectively because the goroutine calls T.Fatal must be called in the same goroutine as the test
- fix error strings that should not be capitalized
- use sort.Strings(...) instead of sort.Sort(sort.StringSlice(...))
- use he status code of Canceled instead of grpc.ErrClientConnClosing which is deprecated
- use use status.Errorf instead of grpc.Errorf which is deprecated

Related #10528 #10438

clientv3/integration/server_shutdown_test.go

@@ -75,16 +75,16 @@ func TestBalancerUnderServerShutdownWatch(t *testing.T) {
 		select {
 		case ev := <-wch:
 			if werr := ev.Err(); werr != nil {
-				t.Fatal(werr)
+				t.Error(werr)
 			}
 			if len(ev.Events) != 1 {
-				t.Fatalf("expected one event, got %+v", ev)
+				t.Errorf("expected one event, got %+v", ev)
 			}
 			if !bytes.Equal(ev.Events[0].Kv.Value, []byte(val)) {
-				t.Fatalf("expected %q, got %+v", val, ev.Events[0].Kv)
+				t.Errorf("expected %q, got %+v", val, ev.Events[0].Kv)
 			}
 		case <-time.After(7 * time.Second):
-			t.Fatal("took too long to receive events")
+			t.Error("took too long to receive events")
 		}
 	}()
 
@@ -350,7 +350,7 @@ func testBalancerUnderServerStopInflightRangeOnRestart(t *testing.T, linearizabl
 		}
 		cancel()
 		if err != nil {
-			t.Fatalf("unexpected error: %v", err)
+			t.Errorf("unexpected error: %v", err)
 		}
 	}()