Use approval testing framework for statement printer test

It's better to store the expected text in a separate file instead of mixed
in together with the kotlin test code. This is for several reasons
- it is easier to read when the expected value is on several lines
- it is easier to update using a diff merge tool
- it is easier to read the VCS history since it will probably change
more frequently than the rest of the test code

kotlin/build.gradle

@@ -17,6 +17,7 @@ dependencies {
     testImplementation 'org.junit.jupiter:junit-jupiter-api:5.3.1'
     testRuntimeOnly 'org.junit.jupiter:junit-jupiter-engine:5.3.1'
     testCompile("org.junit.jupiter:junit-jupiter-params:5.4.2")
+    testCompile 'com.approvaltests:approvaltests:3.2.0'
 }
 
 test {