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/equalsIgnoreCase.kt

@@ -0,0 +1,12 @@
+import kotlin.test.*
+
+
+
+fun box() {
+    assertFalse("sample".equals("Sample", ignoreCase = false))
+    assertTrue("sample".equals("Sample", ignoreCase = true))
+    assertFalse("sample".equals(null, ignoreCase = false))
+    assertFalse("sample".equals(null, ignoreCase = true))
+    assertTrue(null.equals(null, ignoreCase = true))
+    assertTrue(null.equals(null, ignoreCase = false))
+}