Allow creating a GradleRunner without specifying the working directory

[lacasseio]

At the moment, we always have to specify a working directory or else the execution fails because of an assertion. We should make our best to provide a temporary directory by default which is deleted upon process exits (deleteOnExits()).

The issue right now is you can quickly create a runner but the API doesn't make it obvious the working directory is required leading to runtime errors. It's more of an annoyance than anything else.

[lacasseio]

Before we can make this useful, we will need a model for build script which we can pass to the methods such as usingBuildScript/usingSettingsFile/usingInitScript. With that, we could run Gradle as follow:

GradleRunner.create(gradleTestKit()).usingBuildScript(this::myBuildDotGradleSupplier).withTasks('build').build()

The runner would create an empty settings.gradle, create a temporary working directory that would be deleted on exit and generate the build.gradle file inside the working directory when the runner executes. We can imagine usingBuildScript and co. taking a Consumer that configures a builder for the build script model itself.