kotlin.test: Make inline methods 'todo' and 'currentStackTrace' @InlineOnly not to loose stack trace.

Move related tests to separate file and verify the line numbers.
#KT-11348 Fixed

libraries/kotlin.test/shared/src/main/kotlin.jvm/kotlin/test/TestAssertionsJVM.kt

@@ -36,11 +36,18 @@ inline fun <reified T : Throwable> assertFailsWith(message: String? = null, noin
  * Comments out a [block] of test code until it is implemented while keeping a link to the code
  * to implement in your unit test output
  */
+@kotlin.internal.InlineOnly
 inline fun todo(@Suppress("UNUSED_PARAMETER") block: () -> Unit) {
-    System.out.println("TODO at " + currentStackTrace()[1])
+    System.out.println("TODO at " + currentStackTrace()[0])
 }
 
-@Suppress("PLATFORM_CLASS_MAPPED_TO_KOTLIN", "NOTHING_TO_INLINE")
+/**
+ * Returns an array of stack trace elements, each representing one stack frame.
+ * The first element of the array (assuming the array is not empty) represents the top of the
+ * stack, which is the place where [currentStackTrace] function was called from.
+ */
+@Suppress("PLATFORM_CLASS_MAPPED_TO_KOTLIN")
+@kotlin.internal.InlineOnly
 inline fun currentStackTrace() = (java.lang.Exception() as java.lang.Throwable).stackTrace
 
 /**

libraries/kotlin.test/shared/src/test/kotlin.jvm/BasicAssertionsJVMTest.kt

@@ -1,4 +1,4 @@
-package kotlinx.testing.tests
+package kotlin.test.tests
 
 import kotlin.test.*
 import org.junit.*
@@ -31,16 +31,4 @@ class BasicAssertionsJVMTest {
             throw IllegalArgumentException()
         })
     }
-
-    @Test
-    fun testToDo() {
-        todo {
-            fail("Shouldn't pass here")
-        }
-    }
-
-    @Test
-    fun testCurrentStackTrace() {
-        assertEquals("BasicAssertionsJVMTest.kt", currentStackTrace()[0].fileName)
-    }
 }

libraries/kotlin.test/shared/src/test/kotlin.jvm/StackTraceJVMTest.kt

@@ -0,0 +1,23 @@
+package kotlin.test.tests
+
+import org.junit.Test
+import kotlin.test.*
+
+// NOTE: These tests verify line numbers of stack frames, so they might be quite fragile to reordering etc
+
+class StackTraceJVMTest {
+
+    @Test
+    fun testCurrentStackTrace() {
+/* <-- line number */ val topFrame = currentStackTrace()[0]
+        assertEquals("StackTraceJVMTest.kt", topFrame.fileName)
+        assertEquals(12, topFrame.lineNumber)
+    }
+
+    @Test
+    fun testToDo() {
+        todo {
+            fail("Shouldn't pass here")
+        }
+    }
+}