[kotlin.test] Add fluent assertions DSL

[kpavlov]

Add fluent assertions DSL:

"Hello" shouldBe "Hello"
"Hello" shouldNotBe 1

withClue({ "Should be equal" }) {
    "Hello" shouldBe "Hello"
}

val err = assertFailsWith<AssertionError> {
    withClue("Should fail") {
        "Hello" shouldBe "Goodbye"
    }
}
assertEquals("Should fail. Expected <Goodbye>, actual <Hello>.", err.message)

val error = assertFailsWith<AssertionError> {
    withClue({ "Should fail" }) {
        "Hello" shouldBe "Goodbye"
    }
}
assertEquals("Should fail. Expected <Goodbye>, actual <Hello>.", error.message)

assertFailsWith<AssertionError>("shouldBe should fail when expected") {
    "Hello" shouldBe "Goodbye"
}