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/testIndent.kt

@@ -0,0 +1,11 @@
+import kotlin.test.*
+
+
+
+fun box() {
+    assertEquals("  ABC\n  123", "ABC\n123".prependIndent("  "))
+    assertEquals("  ABC\n  \n  123", "ABC\n\n123".prependIndent("  "))
+    assertEquals("  ABC\n  \n  123", "ABC\n \n123".prependIndent("  "))
+    assertEquals("  ABC\n   \n  123", "ABC\n   \n123".prependIndent("  "))
+    assertEquals("  ", "".prependIndent("  "))
+}