[Native][tests] Minor: Add check to ensure all custom KLIBs are valid

native/native.tests/tests/org/jetbrains/kotlin/konan/blackboxtest/support/settings/TestProcessSettings.kt

@@ -64,7 +64,14 @@ internal enum class TestMode(private val description: String) {
  * The set of custom (external) klibs that should be passed to the Kotlin/Native compiler.
  */
 @JvmInline
-internal value class CustomKlibs(val klibs: Set<File>)
+internal value class CustomKlibs(val klibs: Set<File>) {
+    init {
+        val invalidKlibs = klibs.filterNot { it.isDirectory || (it.isFile && it.extension == "klib") }
+        assertTrue(invalidKlibs.isEmpty()) {
+            "There are invalid KLIBs that should be passed for the Kotlin/Native compiler: ${klibs.joinToString { "[$it]" }}"
+        }
+    }
+}
 
 /**
  * Whether to force [TestKind.STANDALONE] for all tests where [TestKind] is assumed to be [TestKind.REGULAR] otherwise: