Add test for converting a string literal to UAST

plugins/uast-kotlin/testData/StringTemplateInClass.kt

@@ -0,0 +1,3 @@
+class Demo {
+    val foo = X.bar("lorem")
+}

plugins/uast-kotlin/tests/KotlinUastApiTest.kt

@@ -1,8 +1,13 @@
 package org.jetbrains.uast.test.kotlin
 
+import org.jetbrains.kotlin.psi.KtLiteralStringTemplateEntry
 import org.jetbrains.uast.UAnnotation
 import org.jetbrains.uast.UFile
+import org.jetbrains.uast.ULiteralExpression
+import org.jetbrains.uast.psi.PsiElementBacked
 import org.jetbrains.uast.test.env.findElementByText
+import org.jetbrains.uast.toUElement
+import org.junit.Assert
 import org.junit.Test
 
 
@@ -17,5 +22,15 @@ class KotlinUastApiTest : AbstractKotlinUastTest() {
             assertEquals(annotation.findAttributeValue("to")?.expression?.evaluate(), 0)
         }
     }
-}
 
+    @Test fun testConvertStringTemplate() {
+        doTest("StringTemplateInClass") { name, file ->
+            val literalExpression = file.findElementByText<ULiteralExpression>("lorem")
+            val psi = (literalExpression as PsiElementBacked).psi!!
+            Assert.assertTrue(psi is KtLiteralStringTemplateEntry)
+            val literalExpressionAgain = psi.toUElement()
+            Assert.assertTrue(literalExpressionAgain is ULiteralExpression)
+
+        }
+    }
+}