go test ./...
go test -run '^TestParsePort$' ./internal/config
go test -count=1 ./...

func TestAdd(t *testing.T) {
	got := Add(1, 2)
	if got != 3 {
		t.Fatalf("got %d, want %d", got, 3)
	}
}

calc.go
calc_test.go

func TestParsePort(t *testing.T) {
	port, err := ParsePort("8080")
	if err != nil {
		t.Fatalf("unexpected error: %v", err)
	}
	if port != 8080 {
		t.Fatalf("got %d, want %d", port, 8080)
	}
}

func TestIsAdult(t *testing.T) {
	tests := []struct {
		name string
		age  int
		want bool
	}{
		{name: "minor", age: 19, want: false},
		{name: "adult", age: 20, want: true},
	}

	for _, tt := range tests {
		t.Run(tt.name, func(t *testing.T) {
			got := IsAdult(tt.age)
			if got != tt.want {
				t.Fatalf("got %v, want %v", got, tt.want)
			}
		})
	}
}

for _, tt := range tests {
	tt := tt // Go 1.21 이하도 안전하게 지원
	t.Run(tt.name, func(t *testing.T) {
		t.Parallel()
		if got := IsAdult(tt.age); got != tt.want {
			t.Fatalf("got %v, want %v", got, tt.want)
		}
	})
}

func mustParse(t *testing.T, raw string) int {
	t.Helper()

	value, err := strconv.Atoi(raw)
	if err != nil {
		t.Fatalf("parse %q: %v", raw, err)
	}
	return value
}