Add test on bytecode text

Test data should be a Kotlin source file with zero or more comments e.g. of
the form: '// 1 INVOKEVIRTUAL'. The test then checks that the generated
bytecode for this file contains exactly one occurrence of the string
'INVOKEVIRTUAL'

compiler/testData/codegen/bytecodeText/statements/when.kt

@@ -0,0 +1,11 @@
+fun z() {}
+
+fun foo(x: Int) {
+    when {
+        x == 21 -> z()
+        x == 42 -> z()
+        else -> {}
+    }
+}
+
+// 0 GETSTATIC