test: use `T.TempDir` to create temporary test directory

[Juneezee]

A testing cleanup.

This pull request replaces ioutil.TempDir with t.TempDir. We can use the T.TempDir function from the testing package to create temporary directory. The directory created by T.TempDir is automatically removed when the test and all its subtests complete.

This saves us at least 2 lines (error check, and cleanup) on every instance, or in some cases adds cleanup that we forgot.

Reference: https://pkg.go.dev/testing#T.TempDir

func TestFoo(t *testing.T) {
	// before
	tmpDir, err := os.MkdirTemp("", "")
	if err != nil {
		t.Fatal(err)
	}
	defer os.RemoveAll(tmpDir)

	// now
	tmpDir := t.TempDir()
}

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