Refactor helper function to run/not run block of common code on specific platform

Use it in StringBuilder tests

libraries/stdlib/test/text/StringBuilderTest.kt

@@ -5,7 +5,9 @@
 
 package test.text
 
-import test.testOnJvm
+import test.TestPlatform
+import test.testOnlyOn
+import test.testExceptOn
 import kotlin.random.Random
 import kotlin.test.*
 import kotlin.text.*
@@ -39,11 +41,11 @@ class StringBuilderTest {
     fun deprecatedAppend() {
         val chars = charArrayOf('a', 'b', 'c', 'd')
         val sb = StringBuilder()
-        testOnJvm {
+        testOnlyOn(TestPlatform.Jvm) {
             sb.append(chars, 1, 2) // Should fail after KT-15220 gets fixed
             assertEquals("bc", sb.toString())
         }
-        if (sb.isEmpty()) {
+        testExceptOn(TestPlatform.Jvm) {
             assertFailsWith<NotImplementedError> {
                 sb.append(chars, 1, 2)
             }
@@ -256,8 +258,9 @@ class StringBuilderTest {
     @Test
     @Suppress("DEPRECATION")
     fun capacityTest() {
-//        assertEquals(100, StringBuilder(100).capacity()) // not implemented in JS
-
+        testExceptOn(TestPlatform.Js) {
+            assertEquals(100, StringBuilder(100).capacity()) // not implemented in JS
+        }
         StringBuilder("string builder from string capacity test").let { sb ->
             assertTrue(sb.capacity() >= sb.length)
         }
@@ -270,7 +273,9 @@ class StringBuilderTest {
             sb.ensureCapacity(1)
             assertTrue(sb.capacity() >= sb.length)
             sb.ensureCapacity(sb.length * 10)
-//            assertTrue(sb.capacity() >= sb.length * 10) // not implemented in JS
+            testExceptOn(TestPlatform.Js) {
+                assertTrue(sb.capacity() >= sb.length * 10) // not implemented in JS
+            }
         }
     }