Make tests resilient to exceptions thrown while reporting exceptions (#19566)

Part of #19542. We protect against the case where `Exception.getMessage`
or `Exception.getStackTrace` throw an exception.

This change makes `testCompilation tests/neg/i8984.scala` fail as
expected. `tests/neg/i8984.scala` is currently disabled (see #19561).

`tests/neg/i8984.scala` will not be fixed in this PR. Making the test
framework more resilient is a priority. Fixing this is the last part of
#19542.

Author: nicolasstucki

Diff for: compiler/test/dotty/tools/vulpix/ParallelTesting.scala

@@ -290,15 +290,25 @@ trait ParallelTesting extends RunnerOrchestration { self =>
 
     /** This callback is executed once the compilation of this test source finished */
     private final def onComplete(testSource: TestSource, reportersOrCrash: Try[Seq[TestReporter]], logger: LoggedRunnable): Unit =
-      reportersOrCrash match {
-        case TryFailure(exn) => onFailure(testSource, Nil, logger, Some(s"Fatal compiler crash when compiling: ${testSource.title}:\n${exn.getMessage}${exn.getStackTrace.map("\n\tat " + _).mkString}"))
-        case TrySuccess(reporters) if !reporters.exists(_.skipped) =>
-          maybeFailureMessage(testSource, reporters) match {
-            case Some(msg) => onFailure(testSource, reporters, logger, Option(msg).filter(_.nonEmpty))
-            case None => onSuccess(testSource, reporters, logger)
+      try
+        reportersOrCrash match
+          case TryFailure(exn) => onFailure(testSource, Nil, logger, Some(s"Fatal compiler crash when compiling: ${testSource.title}:\n${exn.getMessage}${exn.getStackTrace.map("\n\tat " + _).mkString}"))
+          case TrySuccess(reporters) if !reporters.exists(_.skipped) =>
+            maybeFailureMessage(testSource, reporters) match {
+              case Some(msg) => onFailure(testSource, reporters, logger, Option(msg).filter(_.nonEmpty))
+              case None => onSuccess(testSource, reporters, logger)
+            }
+          case _ =>
+      catch case ex: Throwable =>
+        echo(s"Exception thrown onComplete (probably by a reporter) in $testSource: ${ex.getClass}")
+        Try(ex.printStackTrace())
+          .recover{ _ =>
+            val trace = ex.getStackTrace.map(_.toString) // compute this first in case getStackTrace throws an exception
+            echo(s"${ex.getClass.getName} message could not be printed due to an exception while computing the message.")
+            if trace.nonEmpty then trace.foreach(echo) else echo(s"${ex.getClass.getName} stack trace is empty.")
           }
-        case _ =>
-      }
+          .getOrElse(echo(s"${ex.getClass.getName} stack trace could not be printed due to an exception while printing the stack trace."))
+        failTestSource(testSource)
 
     /**
      * Based on the reporters obtained after the compilation, determines if this test has failed.