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/src/test/kotlin/StatementPrinterTests.exampleStatement$kotlin.approved.txt

@@ -0,0 +1,6 @@
+Statement for BigCo
+  Hamlet: $650.00 (55 seats)
+  As You Like It: $580.00 (35 seats)
+  Othello: $500.00 (40 seats)
+Amount owed is $1,730.00
+You earned 47 credits

kotlin/src/test/kotlin/StatementPrinterTests.kt

@@ -1,17 +1,12 @@
-import org.junit.jupiter.api.Assertions.assertEquals
+
 import org.junit.jupiter.api.Assertions.assertThrows
 import org.junit.jupiter.api.Test
+import org.approvaltests.Approvals.verify
 
 class StatementPrinterTests {
 
     @Test
     internal fun exampleStatement() {
-        val expected = "Statement for BigCo\n" +
-                "  Hamlet: \$650.00 (55 seats)\n" +
-                "  As You Like It: \$580.00 (35 seats)\n" +
-                "  Othello: \$500.00 (40 seats)\n" +
-                "Amount owed is \$1,730.00\n" +
-                "You earned 47 credits\n"
 
         val plays = mapOf(
             "hamlet" to Play("Hamlet", "tragedy"),
@@ -30,7 +25,7 @@ class StatementPrinterTests {
         val statementPrinter = StatementPrinter()
         val result = statementPrinter.print(invoice, plays)
 
-        assertEquals(expected, result)
+        verify(result)
     }
 
     @Test