[REVIEW] better C++ test parameter management (#3088)

* draft 1 of better test parameter specification

* refactor using variadic macros; move fil enums to own namespace

* changelog; fixed fil.pyx enum import

* simpler FIL_TEST_PARAMS macro, remove the ::enums:: changes

* leaner change

* renamed struct responsible for non default FIL test parameters

* style

CHANGELOG.md

@@ -19,6 +19,7 @@
 - PR #3044: Move leftover `linalg` and `stats` to RAFT namespaces
 - PR #3067: Deleting prims moved to RAFT and updating header paths
 - PR #3074: Reducing dask coordinate descent test runtime
+- PR #3088: More readable and robust FIL C++ test management
 - PR #3052: Speeding up MNMG KNN Cl&Re testing
 - PR #3115: Speeding up MNMG UMAP testing
 - PR #3112: Speed test_array

cpp/include/cuml/fil/fil.h

@@ -71,6 +71,9 @@ enum output_t {
   /** output class label: either apply threshold to the output of the previous stage (for binary classification),
       or select the class with the most votes to get the class label (for multi-class classification).  */
   CLASS = 0x100,
+  SIGMOID_CLASS = SIGMOID | CLASS,
+  AVG_CLASS = AVG | CLASS,
+  AVG_SIGMOID_CLASS = AVG | SIGMOID | CLASS,
 };
 
 /** storage_type_t defines whether to import the forests as dense or sparse */