Create a testing tool

People all have their own way of integration-testing Javalin apps. Maybe we should create an official package with a http-client and a set of useful methods?

The main goal is enabling people (and the Javalin project itself), to setup simple tests like this:

@Test
fun `error-mapper works for 500`() = TestUtil.test { app, http ->
    app.get("/exception") { throw RuntimeException() }
    app.error(500) { ctx -> ctx.result("Custom 500 page") }
    assertThat(http.getBody("/exception")).isEqualTo("Custom 500 page")
}

[alexey-wg2]

I can take this 😃

[oharaandrew314]

@tipsy I agree it’s probably best to include it as a submodule. I had some work in progress to incorporate Rest Assured into the test project, but I think it would be pretty simple to add it to the JUnit Rule if we desire.