Disallow duplicate names within the same context

[npryce]

Either tests with the same name, subcontexts with the same name, or a test with the same name as a subcontext.

    @org.junit.jupiter.api.Test
    fun `names are unique within a context`() {
        assertFailsWith<IllegalArgumentException>("tests") {
            rootContext<Unit> {
                test("nameclash") {}
                test("nameclash") {}
            }.buildNode()
        }
        assertFailsWith<IllegalArgumentException>("contexts") {
            rootContext<Unit> {
                context("nameclash") {}
                context("nameclash") {}
            }.buildNode()
        }
        assertFailsWith<IllegalArgumentException>("context and test") {
            rootContext<Unit> {
                context("nameclash") {}
                test("nameclash") {}
            }.buildNode()
        }
    }