client_test: Keep NEO srv logs if test fails

If a test fails we may want to invest further why this test fails and
reading the NEO server log files can be an efficient way for these
investigations. Therefore we should keep log files if a test fails.

go/neo/client_test.go

@@ -423,7 +423,13 @@ func withNEOSrv(t *testing.T, f func(t *testing.T, nsrv NEOSrv), optv ...tOption
 		t.Helper()
 		X := xtesting.FatalIf(t)
 		work, err := ioutil.TempDir("", "neo"); X(err)
-		defer os.RemoveAll(work)
+		defer func() {
+			if t.Failed() {
+				t.Logf("NEO server log files reside in %s", work)
+			} else {
+				os.RemoveAll(work)
+			}
+		}()
 
 		f(work)
 	}