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/dropWhile.kt

@@ -0,0 +1,13 @@
+import kotlin.test.*
+
+// helper predicates available on both platforms
+fun Char.isAsciiDigit() = this in '0'..'9'
+fun Char.isAsciiLetter() = this in 'A'..'Z' || this in 'a'..'z'
+fun Char.isAsciiUpperCase() = this in 'A'..'Z'
+
+fun box() {
+    val data = "ab1cd2"
+    assertEquals("1cd2", data.dropWhile { it.isAsciiLetter() })
+    assertEquals("", data.dropWhile { true })
+    assertEquals("ab1cd2", data.dropWhile { false })
+}