Fix snippet test mismatch between hotspot and native test (#1797)

pkl-core/src/test/files/LanguageSnippetTests/output/errors/traceThenIterateUndefinedMember.err

@@ -1,3 +1,4 @@
+pkl: TRACE: items = new Listing { ? } (file:///$snippetsDir/input/errors/traceThenIterateUndefinedMember.pkl)
 –– Pkl Error ––
 Tried to read property `undefinedValue` but its value is undefined.
 
@@ -24,4 +25,3 @@ at pkl.base#Module.output.text (pkl:base)
 xxx | if (renderer is BytesRenderer) renderer.renderDocument(value) else text.encodeToBytes("UTF-8")
                                                                          ^^^^
 at pkl.base#Module.output.bytes (pkl:base)
-pkl: TRACE: items = new Listing { ? } (file:///$snippetsDir/input/errors/traceThenIterateUndefinedMember.pkl)

pkl-core/src/test/kotlin/org/pkl/core/LanguageSnippetTestsEngine.kt

@@ -223,8 +223,11 @@ class LanguageSnippetTestsEngine : AbstractLanguageSnippetTestsEngine() {
 
     val stderr = logWriter.toString().withUnixLineEndings()
 
+    // match native CLI: if an error happens, any WARN or TRACE gets emitted first, then the
+    // error message
+    val effectiveOutput = if (success) output + stderr else stderr + output
     return (success && stderr.isBlank()) to
-      (output + stderr).stripFilePaths().stripWebsite().stripStdlibLocationSha()
+      effectiveOutput.stripFilePaths().stripWebsite().stripStdlibLocationSha()
   }
 }