Reduce validation test suite execution time by removing -workers auto

* `-workers auto` was being used alongside `-workers 2`.
* In some environments, `-workers auto` appears to resolve to `-workers 1`, even on multicore systems (suggesting core detection may be unreliable).
* Most validation tests have small state spaces and gain little benefit from higher worker counts, making `-workers auto` unnecessary overhead.

Author: lemmy

testgen/spec.py

@@ -216,7 +216,7 @@ def prepare_testcases(feature_filter, symmetry, anomalous):
     # CMD options combinations
     # So far only `-workers` option varies
     cmd_options_ref = ['-workers', '1']
-    workers_options = ['2', 'auto']
+    workers_options = ['2']
     cmd_options_variants = [(f'-workers {opt}', ['-workers', opt]) for opt in workers_options]
 
     new_testcases = []