all: use T.TempDir to create temporary test directories (#24633)

This commit replaces ioutil.TempDir with t.TempDir in tests. The
directory created by t.TempDir is automatically removed when the test
and all its subtests complete.

Prior to this commit, temporary directory created using ioutil.TempDir
had to be removed manually by calling os.RemoveAll, which is omitted in
some tests. The error handling boilerplate e.g.

	defer func() {
		if err := os.RemoveAll(dir); err != nil {
			t.Fatal(err)
		}
	}

is also tedious, but t.TempDir handles this for us nicely.

Reference: https://pkg.go.dev/testing#T.TempDir
Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

cmd/geth/genesis_test.go

@@ -18,7 +18,6 @@ package main
 
 import (
 	"io/ioutil"
-	"os"
 	"path/filepath"
 	"testing"
 )
@@ -73,8 +72,7 @@ var customGenesisTests = []struct {
 func TestCustomGenesis(t *testing.T) {
 	for i, tt := range customGenesisTests {
 		// Create a temporary data directory to use and inspect later
-		datadir := tmpdir(t)
-		defer os.RemoveAll(datadir)
+		datadir := t.TempDir()
 
 		// Initialize the data directory with the custom genesis block
 		json := filepath.Join(datadir, "genesis.json")