fix: stop tagging "test exception" testcases as synthetic skip

"test exception" is not a synthetic placeholder emitted by any test
framework — JUnit 4, JUnit Platform, Gradle's adapters, Spock and Kotest
all reserve "initializationError"/"executionError" for that purpose, not
generic English names. In dd-trace-java the entries observed in CI came
from a real Spock feature method (`def "test exception"()` in
HttpServerTest.groovy), not a framework synthetic. Tagging that name
unconditionally silently masks real pass/fail outcomes for any
Spock/JUnit 5 @DisplayName/Kotest test that happens to use it.

Drop "test exception" from JUnitReport.tagSyntheticFailures and document
the criteria for future synthetic entries (must be framework-emitted,
must link to the source pinned to a release tag). Annotate the two
remaining entries — initializationError (JUnit 4 + Gradle) and
executionError (Gradle) — with the canonical source locations.

Author: bric3

.gitlab/collect-result/JUnitReport.java

@@ -66,15 +66,34 @@ boolean normalizeStableTestNames() {
     return changed;
   }
 
+  // Tags framework-emitted synthetic testcases so Test Optimization does not treat them as real
+  // failures. Any name added here must be a placeholder that the test framework/runner produces
+  // itself — never a name that a user-authored test could legitimately have. New entries must
+  // include a link to the framework/library source that emits the literal, pinned to a release
+  // tag (not a commit hash). Frameworks/libraries to audit when adding a synthetic: JUnit 4
+  // (org.junit.internal.runners.ErrorReportingRunner), JUnit Platform / Vintage / Jupiter
+  // engines, Gradle's JUnit/JUnitPlatform test event adapters, TestNG, Spock, Kotest, Maven
+  // Surefire/Failsafe. Generic English names (e.g. "test exception") must not be added — any
+  // Spock feature method (`def "..."()`), JUnit 5 `@DisplayName`, or Kotest spec can produce
+  // them, which would silently mask real pass/fail outcomes.
   void tagSyntheticFailures() {
     Map<String, List<Element>> initializationErrorsByClassname = new LinkedHashMap<>();
     for (var testcase : testcases()) {
       var name = testcase.getAttribute("name");
+      // JUnit 4 emits "initializationError" when the test class cannot be loaded, has no runnable
+      // methods, or its @BeforeClass fails — see
+      // https://github.com/junit-team/junit4/blob/r4.13.2/src/main/java/org/junit/internal/runners/ErrorReportingRunner.java#L83
+      // Gradle's JUnit Platform listener reuses the same name for synthetic container failures —
+      // see
+      // https://github.com/gradle/gradle/blob/v9.5.0/platforms/jvm/testing-jvm-infrastructure/src/main/java/org/gradle/api/internal/tasks/testing/junitplatform/JUnitPlatformTestExecutionListener.java#L426
       if ("initializationError".equals(name)) {
         initializationErrorsByClassname
             .computeIfAbsent(testcase.getAttribute("classname"), ignored -> new ArrayList<>())
             .add(testcase);
-      } else if ("executionError".equals(name) || "test exception".equals(name)) {
+      } else if ("executionError".equals(name)) {
+        // Gradle's JUnit Platform listener emits "executionError" for container nodes whose
+        // descendants already started before the failure — see
+        // https://github.com/gradle/gradle/blob/v9.5.0/platforms/jvm/testing-jvm-infrastructure/src/main/java/org/gradle/api/internal/tasks/testing/junitplatform/JUnitPlatformTestExecutionListener.java#L426
         addFinalStatusProperty(testcase, "skip", MissingPropertiesPlacement.APPEND_TO_TESTCASE);
       }
     }