Add custom test logger that writes to stderr

src/commonTest/kotlin/fr/acinq/lightning/tests/utils/TestLogger.kt

@@ -16,16 +16,25 @@
 
 package fr.acinq.lightning.tests.utils
 
-import co.touchlab.kermit.CommonWriter
-import co.touchlab.kermit.NoTagFormatter
-import co.touchlab.kermit.Severity
-import co.touchlab.kermit.loggerConfigInit
+import co.touchlab.kermit.*
 import fr.acinq.lightning.logging.LoggerFactory
 
+expect fun printToStderr(msg: String)
+
+class StderrWriter(val formatter: MessageStringFormatter) : LogWriter() {
+    override fun log(severity: Severity, message: String, tag: String, throwable: Throwable?) {
+        printToStderr(formatter.formatMessage(severity, Tag(tag), Message(message)))
+        throwable?.let {
+            printToStderr(it.toString())
+            printToStderr(it.stackTraceToString())
+        }
+    }
+}
+
 val testLoggerFactory: LoggerFactory by lazy {
     LoggerFactory(
         loggerConfigInit(
-            logWriters = arrayOf(CommonWriter(NoTagFormatter)),
+            logWriters = arrayOf(StderrWriter(NoTagFormatter)),
             minSeverity = Severity.Info
         )
     )

src/jvmTest/kotlin/fr/acinq/lightning/tests/utils/TestLogger.jvm.kt

@@ -0,0 +1,5 @@
+package fr.acinq.lightning.tests.utils
+
+actual fun printToStderr(msg: String) {
+    System.err.println(msg)
+}

src/nativeTest/kotlin/fr/acinq/lightning/tests/utils/TestLogger.native.kt

@@ -0,0 +1,12 @@
+package fr.acinq.lightning.tests.utils
+
+import kotlinx.cinterop.ExperimentalForeignApi
+import platform.posix.fflush
+import platform.posix.fprintf
+
+@OptIn(ExperimentalForeignApi::class)
+actual fun printToStderr(msg: String) {
+    val stderr = platform.posix.fdopen(2, "w")
+    fprintf(stderr, msg + "\n")
+    fflush(stderr)
+}