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/StringBuilderTest/append.kt

@@ -0,0 +1,9 @@
+import kotlin.test.*
+
+
+fun box() {
+    // this test is needed for JS implementation
+    assertEquals("em", buildString {
+        append("element", 2, 4)
+    })
+}

backend.native/tests/external/stdlib/text/StringBuilderTest/appendMany.kt

@@ -0,0 +1,8 @@
+import kotlin.test.*
+
+
+fun box() {
+    assertEquals("a1", StringBuilder().append("a", "1").toString())
+    assertEquals("a1", StringBuilder().append("a", 1).toString())
+    assertEquals("a1", StringBuilder().append("a", StringBuilder().append("1")).toString())
+}

backend.native/tests/external/stdlib/text/StringBuilderTest/asCharSequence.kt

@@ -0,0 +1,17 @@
+import kotlin.test.*
+
+
+fun box() {
+    val original = "Some test string"
+    val sb = StringBuilder(original)
+    val result = sb.toString()
+    val cs = sb as CharSequence
+
+    assertEquals(result.length, cs.length)
+    assertEquals(result.length, sb.length)
+    for (index in result.indices) {
+        assertEquals(result[index], sb[index])
+        assertEquals(result[index], cs[index])
+    }
+    assertEquals(result.substring(2, 6), cs.subSequence(2, 6).toString())
+}

backend.native/tests/external/stdlib/text/StringBuilderTest/constructors.kt

@@ -0,0 +1,24 @@
+import kotlin.test.*
+
+
+fun box() {
+    StringBuilder().let { sb ->
+        assertEquals(0, sb.length)
+        assertEquals("", sb.toString())
+    }
+
+    StringBuilder(16).let { sb ->
+        assertEquals(0, sb.length)
+        assertEquals("", sb.toString())
+    }
+
+    StringBuilder("content").let { sb ->
+        assertEquals(7, sb.length)
+        assertEquals("content", sb.toString())
+    }
+
+    StringBuilder(StringBuilder("content")).let { sb ->
+        assertEquals(7, sb.length)
+        assertEquals("content", sb.toString())
+    }
+}

backend.native/tests/external/stdlib/text/StringBuilderTest/stringBuild.kt

@@ -0,0 +1,10 @@
+import kotlin.test.*
+
+
+fun box() {
+    val s = buildString {
+        append("a")
+        append(true)
+    }
+    assertEquals("atrue", s)
+}