backend/tests: Add blackbox tests from Kotlin JVM

Added tests from testData/codegen/box directory. There are blackbox tests
in other directories and they are to be added.

backend.native/tests/external/codegen/blackbox/unit/UnitValue.kt

@@ -0,0 +1,8 @@
+// TODO: muted automatically, investigate should it be ran for JS or not
+// IGNORE_BACKEND: JS
+
+fun foo() {}
+
+fun box(): String {
+  return if (foo() == Unit) "OK" else "Fail"
+}

backend.native/tests/external/codegen/blackbox/unit/closureReturnsNullableUnit.kt

@@ -0,0 +1,8 @@
+fun isNull(x: Unit?) = x == null
+
+fun box(): String {
+    val closure: () -> Unit? = { null }
+    if (!isNull(closure())) return "Fail 1"
+
+    return "OK"
+}

backend.native/tests/external/codegen/blackbox/unit/ifElse.kt

@@ -0,0 +1,31 @@
+class A (val p: String, p1: String, p2: String) {
+
+    var cond1: String = ""
+
+    var cond2: String = ""
+
+    val prop1 = if (cond1(p)) p1 else null
+
+    val prop2 = if (cond2(p)) p2 else null;
+
+
+    fun cond1(p: String): Boolean {
+        cond1 = "cond1"
+        return p == "test"
+    }
+
+    fun cond2(p: String): Boolean {
+        cond2 = "cond2"
+        return p == "test"
+    }
+}
+
+fun box(): String {
+    val a = A("test", "OK", "fail")
+
+    if (a.cond1 != "cond1") return "fail 2 : ${a.cond1}"
+
+    if (a.cond2 != "cond2") return "fail 3 : ${a.cond2}"
+
+    return "OK"
+}

backend.native/tests/external/codegen/blackbox/unit/kt3634.kt

@@ -0,0 +1,8 @@
+val c = Unit
+val d = c
+
+fun box(): String {
+    c
+    d
+    return "OK"
+}

backend.native/tests/external/codegen/blackbox/unit/kt4212.kt

@@ -0,0 +1,23 @@
+// TODO: muted automatically, investigate should it be ran for JS or not
+// IGNORE_BACKEND: JS
+
+fun foo(): Any? = bar()
+
+fun bar() {}
+
+fun baz(): Any? {
+    return bar()
+}
+
+fun quux(): Unit? = bar()
+
+fun box(): String {
+    foo()
+
+    if (foo() != Unit) return "Fail 1"
+    if (foo() != bar()) return "Fail 2"
+    if (bar() != baz()) return "Fail 3"
+    if (baz() != quux()) return "Fail 4"
+
+    return "OK"
+}

backend.native/tests/external/codegen/blackbox/unit/kt4265.kt

@@ -0,0 +1,14 @@
+fun <T : Any, R> T.let(f: (T) -> R): R = f(this)
+
+fun box(): String {
+    val o: String? = null
+
+    var state = 0
+
+    o?.let {
+        state = 1
+    } ?: ({ state = 2 })()
+
+    if (state != 2) return "Fail: $state"
+    return "OK"
+}

backend.native/tests/external/codegen/blackbox/unit/nullableUnit.kt

@@ -0,0 +1,20 @@
+fun isNull(x: Unit?) = x == null
+
+fun <T : Any> isNullGeneric(x: T?) = x == null
+
+fun deepIsNull0(x: Unit?) = isNull(x)
+fun deepIsNull(x: Unit?) = deepIsNull0(x)
+
+fun box(): String {
+    if (!isNull(null)) return "Fail 1"
+
+    val x: Unit? = null
+    if (!isNull(x)) return "Fail 2"
+
+    val y = x
+    if (!isNullGeneric(y)) return "Fail 3"
+
+    if (!deepIsNull(x ?: null)) return "Fail 4"
+
+    return "OK"
+}

backend.native/tests/external/codegen/blackbox/unit/nullableUnitInWhen1.kt

@@ -0,0 +1,12 @@
+fun foo() {}
+
+fun box(): String {
+    when ("A") {
+        "B" -> foo()
+        else -> null
+    }
+
+    foo()
+
+    return "OK"
+}

backend.native/tests/external/codegen/blackbox/unit/nullableUnitInWhen2.kt

@@ -0,0 +1,12 @@
+fun foo() {}
+
+fun box(): String {
+    when ("A") {
+        "B" -> null
+        else -> foo()
+    }
+
+    foo()
+
+    return "OK"
+}

backend.native/tests/external/codegen/blackbox/unit/nullableUnitInWhen3.kt

@@ -0,0 +1,12 @@
+fun foo() {}
+
+fun box(): String {
+    val x = when ("A") {
+        "B" -> foo()
+        else -> null
+    }
+
+    foo()
+    
+    return if (x == null) "OK" else "Fail"
+}

backend.native/tests/external/codegen/blackbox/unit/unitClassObject.kt

@@ -0,0 +1,11 @@
+fun box(): String {
+    Unit
+
+    val a = Unit
+    val b = Unit
+    if (a != b) return "Fail a != b"
+
+    if (Unit != Unit) return "Fail Unit != Unit"
+
+    return "OK"
+}