Minor refactorings in legacy codegen tests

Use loadFile() + getPrefix() instead of loadFile(String)

compiler/testData/codegen/properties/kt2677.kt

@@ -0,0 +1,13 @@
+class U<T>
+
+open class WeatherReport
+{
+    public open var forecast: U<String> = U<String>()
+}
+
+open class DerivedWeatherReport() : WeatherReport()
+{
+    public override var forecast: U<String>
+        get() = super.forecast
+        set(newv: U<String>) { super.forecast = newv }
+}