fix errors like parse error still printed in color when -plaintext is set

codeGenCpu6502/test/Dummies.kt

@@ -68,6 +68,7 @@ internal class ErrorReporterForTests(private val throwExceptionAtReportIfErrors:
 
     override fun noErrors(): Boolean  = errors.isEmpty()
     override fun noErrorForLine(position: Position) = !errors.any { ":${position.line}:" in it }
+    override fun print_single_error(errormessage: String) { /* prints nothing in tests */ }
 
     override fun report() {
         infos.forEach { println("UNITTEST COMPILATION REPORT: INFO: $it") }