Make assertions worker-friendly. (#2152)

backend.native/tests/runtime/workers/worker0.kt

@@ -12,7 +12,10 @@ import kotlin.native.concurrent.*
 @Test fun runTest() {
     val worker = Worker.start()
     val future = worker.execute(TransferMode.SAFE, { "Input" }) {
-        input -> input + " processed"
+        input ->
+        assertEquals(1, 1)
+        assertFailsWith<AssertionError> { assertEquals(1, 2) }
+        input + " processed"
     }
     future.consume {
         result -> println("Got $result")

backend.native/tests/testing/assertions.kt

@@ -172,11 +172,5 @@ private fun checkFailedAssertion(assertion: () -> Unit) {
 
 @Suppress("INVISIBLE_MEMBER")
 private fun withDefaultAsserter(block: () -> Unit) {
-    val current = overrideAsserter(DefaultAsserter)
-    try {
-        block()
-    }
-    finally {
-        overrideAsserter(current)
-    }
+    block()
 }