Minor, restore abiStabilityUnstableWithOldBackend CLI test

Add -Xuse-old-backend to make it pass with LV=1.5 where JVM IR is
enabled by default.

compiler/testData/cli/jvm/abiStabilityUnstableWithOldBackend.args

@@ -0,0 +1,5 @@
+$TESTDATA_DIR$/simple.kt
+-d
+$TEMP_DIR$
+-Xabi-stability=unstable
+-Xuse-old-backend

compiler/testData/cli/jvm/abiStabilityUnstableWithOldBackend.out

@@ -0,0 +1,2 @@
+error: -Xabi-stability=unstable is not supported in the old JVM backend
+COMPILATION_ERROR

compiler/tests-gen/org/jetbrains/kotlin/cli/CliTestGenerated.java

@@ -32,6 +32,11 @@ public class CliTestGenerated extends AbstractCliTest {
             runTest("compiler/testData/cli/jvm/abiStabilityIncorrectValue.args");
         }
 
+        @TestMetadata("abiStabilityUnstableWithOldBackend.args")
+        public void testAbiStabilityUnstableWithOldBackend() throws Exception {
+            runTest("compiler/testData/cli/jvm/abiStabilityUnstableWithOldBackend.args");
+        }
+
         public void testAllFilesPresentInJvm() throws Exception {
             KtTestUtil.assertAllTestsPresentByMetadataWithExcluded(this.getClass(), new File("compiler/testData/cli/jvm"), Pattern.compile("^(.+)\\.args$"), null, false);
         }