tests: get test name from testing.T (#22941)

There were 2 TODOs about that fix after Golang 1.8 release.
It's here for 3 years already, so now should be the right time.

tests/difficulty_test.go

@@ -79,12 +79,12 @@ func TestDifficulty(t *testing.T) {
 	dt.config("difficulty.json", mainnetChainConfig)
 
 	dt.walk(t, difficultyTestDir, func(t *testing.T, name string, test *DifficultyTest) {
-		cfg := dt.findConfig(name)
+		cfg := dt.findConfig(t)
 		if test.ParentDifficulty.Cmp(params.MinimumDifficulty) < 0 {
 			t.Skip("difficulty below minimum")
 			return
 		}
-		if err := dt.checkFailure(t, name, test.Run(cfg)); err != nil {
+		if err := dt.checkFailure(t, test.Run(cfg)); err != nil {
 			t.Error(err)
 		}
 	})