Partest doesn't report warnings if multi-round compilation

[som-snytt]

Reproduction steps

Scala version: 2.13.10

partest jvm/t7253

ought to report the warnings which show the code is not valid.

I tried to tweak it to at least put warnings in the transcript under --verbose.

% scalac t7253/test.scala
% WARNINGS!
% test.scala:14: warning: method without a parameter list overrides a method with a single empty one
  def show: Unit = {
      ^
test.scala:28: warning: comparing values of types Int and String using `==` will always yield false
    (opcode == "INVOKEVIRTUAL") || (opcode == "INVOKEINTERFACE")
            ^
test.scala:28: warning: comparing values of types Int and String using `==` will always yield false
    (opcode == "INVOKEVIRTUAL") || (opcode == "INVOKEINTERFACE")
                                           ^

Problem

Posting a ticket as reminder to return to this later.

Alternatively, partest DirectCompiler could turn on -Werror, perhaps under --verbose.