Test: animation, avoid segfault in keylist unit tests

Replace `EXPECT_NE(column, nullptr)` with `ASSERT_NE(column, nullptr)` to
abort the test on failure. With `EXPECT_NE`, the test would continue onto
the next like, which accesses `column->cfra` and would segfault.

No functional changes to the tests. Just better reporting of failures.

source/blender/editors/animation/keyframes_keylist_test.cc

@@ -51,7 +51,7 @@ static void assert_act_key_column(const ActKeyColumn *column,
                                   const std::optional<float> expected_frame)
 {
   if (expected_frame.has_value()) {
-    EXPECT_NE(column, nullptr);
+    ASSERT_NE(column, nullptr);
     EXPECT_NEAR(column->cfra, *expected_frame, KEYLIST_NEAR_ERROR);
   }
   else {