Minor refactorings in legacy codegen tests

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

compiler/testData/codegen/properties/kt1846.kt

@@ -0,0 +1,4 @@
+class A(private var v1: String) {
+    private var v2 = v1
+    override fun toString(): String { return "A[v1=$v1,v2=$v2]" }
+}