Se eu colocar essas 2 linhas de código: System.setProperty("webdriver.gecko.driver", "c:\geckodriver.exe"); driver = new FirefoxDriver(); em cada teste sem o before e o after funciona, após implementar esses métodos meu código ficou como abaixo e parou de funcionar.

package br.com.caelum.teste;

import static org.junit.Assert.assertTrue;

import org.junit.After;
import org.junit.Before;
import org.junit.jupiter.api.Test;
import org.openqa.selenium.By;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.WebElement;
import org.openqa.selenium.firefox.FirefoxDriver;

public class UsuariosSystemTest {

    private FirefoxDriver driver;

    @Before
    public void inicializa() {
        System.setProperty("webdriver.gecko.driver", "c:\\geckodriver.exe");
        driver = new FirefoxDriver();
    }

    @Test
    public void deveAdicionarUmUsuario() {
        driver.get("http://localhost:8080/usuarios/new");

        WebElement nome = driver.findElement(By.name("usuario.nome"));
        WebElement email = driver.findElement(By.name("usuario.email"));

        nome.sendKeys("Ronaldo Luiz de Albuquerque");
        email.sendKeys("ronaldo2009@terra.com.br");

        WebElement botaoSalvar = driver.findElement(By.id("btnSalvar"));
        botaoSalvar.click();

        boolean achouNome = driver.getPageSource().contains("Ronaldo Luiz de Albuquerque");
        boolean achouEmail = driver.getPageSource().contains("ronaldo2009@terra.com.br");

        assertTrue(achouNome);
        assertTrue(achouEmail);

    }

    @Test
    public void naoDeveAdicionarUmUsuario() {
        driver.get("http://localhost:8080/usuarios/new");

        WebElement email = driver.findElement(By.name("usuario.email"));

        email.sendKeys("ronaldo2009@terra.com.br");

        WebElement botaoSalvar = driver.findElement(By.id("btnSalvar"));
        botaoSalvar.click();

        assertTrue(driver.getPageSource().contains("Nome obrigatorio!"));

    }

    @Test
    public void naoDeveAdicionarUmUsuarioSemNomeOuEmail() {
        driver.get("http://localhost:8080/usuarios/new");

        WebElement botaoSalvar = driver.findElement(By.id("btnSalvar"));
        botaoSalvar.click();

        boolean semNome = driver.getPageSource().contains("Nome obrigatorio!");
        boolean semEmail = driver.getPageSource().contains("E-mail obrigatorio!");

        assertTrue(semNome);
        assertTrue(semEmail);

    }

    @After
    public void fecha() {
        driver.close();
    }
}