Add tests for event type rule identification

Add tests that verify that the event type identification functionality
is working. Notable changes:

 - Modify falco_test.py to additionally check for warnings when loading
   any set of rules and verify that the event types for each rule match
   expected values. This is controlled by the new multiplex fields
   "rules_warning" and "rules_events".

 - Instead of starting with an empty falco_tests.yaml from scratch from
   the downloaded trace files, use a checked-in version which defines
   two tests:
     - Loading the checked-in falco_rules.yaml and verify that no rules
       have warnings.
     - A sample falco_rules_warnings.yaml that has ~30 different
       mutations of rule filtering expressions. The test verifies for each
       rule whether or not the rule should result in a warning and what the
       extracted event types are.
   The generated tests from the trace files are appended to this file.

- Add an empty .scap file to use with the above tests.

test/run_regression_tests.sh

@@ -36,7 +36,7 @@ EOF
 }
 
 function prepare_multiplex_file() {
-    echo "trace_files: !mux" > $MULT_FILE
+    cp $SCRIPTDIR/falco_tests.yaml.in $MULT_FILE
 
     prepare_multiplex_fileset traces-positive True Warning False
     prepare_multiplex_fileset traces-negative False Warning True