Extracted complex string concatenation bytecode test case into separate test data file.

compiler/testData/codegen/bytecodeText/stringOperations/multipleNestedConcat.kt

@@ -0,0 +1,10 @@
+// Uses 3 StringBuilders:
+// - In return expression
+// - In argument to 1st call to foo()
+// - In argument to 2nd call to foo() inside string literal
+fun test(s1: String, s2: String, s3: String): String {
+    fun foo(s: String) = s
+    return "foo: " + foo(s1 + s2 + " ${foo("\${s3.length} = ${s3.length}")}")
+}
+
+// 3 NEW java/lang/StringBuilder