tests: Copy stdlib tests from Kotlin/JVM

This patch copies Kotlin/JVM stdlib tests (libraries/stdlib/test)
excepting ones with explicit java dependencies. It also transforms
the tests to use them as box-tests.

backend.native/tests/external/stdlib/text/StringTest/joinToString.kt

@@ -0,0 +1,17 @@
+import kotlin.test.*
+
+
+
+fun box() {
+    val data = "abcd".toList()
+    val result = data.joinToString("_", "(", ")")
+    assertEquals("(a_b_c_d)", result)
+
+    val data2 = "verylongstring".toList()
+    val result2 = data2.joinToString("-", "[", "]", 11, "oops")
+    assertEquals("[v-e-r-y-l-o-n-g-s-t-r-oops]", result2)
+
+    val data3 = "a1/b".toList()
+    val result3 = data3.joinToString() { it.toUpperCase().toString() }
+    assertEquals("A, 1, /, B", result3)
+}