Make sure One Shot Optimizer always returns the same suggestion (#759)

Make sure the `OneShotOptimizer.suggest()` call always returns the same
configuration.

Changes related to `MockEnv` determinism moved to #769 

Closes #755

---------

Co-authored-by: Brian Kroth <bpkroth@users.noreply.github.com>

mlos_bench/mlos_bench/optimizers/one_shot_optimizer.py

@@ -18,7 +18,7 @@ _LOG = logging.getLogger(__name__)
 
 class OneShotOptimizer(MockOptimizer):
     """
-    Mock optimizer that proposes a single configuration and returns.
+    No-op optimizer that proposes a single configuration and returns.
     Explicit configs (partial or full) are possible using configuration files.
     """
 
@@ -33,6 +33,14 @@ class OneShotOptimizer(MockOptimizer):
         _LOG.info("Run a single iteration for: %s", self._tunables)
         self._max_iter = 1  # Always run for just one iteration.
 
+    def suggest(self) -> TunableGroups:
+        """
+        Always produce the same (initial) suggestion.
+        """
+        tunables = super().suggest()
+        self._start_with_defaults = True
+        return tunables
+
     @property
     def supports_preload(self) -> bool:
         return False