Polish test result running and reporting (#738)

Changes:
* Move class `TestResults` to package `org.pkl.core`, because it is a public class (it's the result of `Evaluator#evaluateTest`)
* Change examples to treat individual examples as assertions in the same test. Previously, they were considered different tests with an incrementing number. This better aligns with how facts are treated.
* Change `TestResults` to be a record, and introduce builders.
* Remove "module" test result section (it is not really a section).
* Add javadoc to `TestResults`
* Formatting fix: prefix ✍️ emoji just like we do the ❌ and ✅ emojis 
* Consider writing examples as failures, not successes. `pkl test` will exit with code 10 if the only failing tests are due to writing examples.

pkl-cli/src/main/kotlin/org/pkl/cli/CliTestRunner.kt

@@ -60,12 +60,14 @@ constructor(
     val evaluator = builder.build()
     evaluator.use {
       var failed = false
+      var isExampleWrittenFailure = true
       val moduleNames = mutableSetOf<String>()
       for ((idx, moduleUri) in sources.withIndex()) {
         try {
           val results = evaluator.evaluateTest(uri(moduleUri), testOptions.overwrite)
           if (!failed) {
             failed = results.failed()
+            isExampleWrittenFailure = results.isExampleWrittenFailure.and(isExampleWrittenFailure)
           }
           SimpleReport().report(results, consoleWriter)
           if (sources.size > 1 && idx != sources.size - 1) {
@@ -101,7 +103,8 @@ constructor(
         }
       }
       if (failed) {
-        throw CliTestException(ErrorMessages.create("testsFailed"))
+        val exitCode = if (isExampleWrittenFailure) 10 else 1
+        throw CliTestException(ErrorMessages.create("testsFailed"), exitCode)
       }
     }
   }