ENH: Create tests for MixedBehaviorProfile as_strategy

[rahulsavani]

This issue is about MixedBehaviorProfile.as_strategy, which turns a behavior profile into a MixedStrategyProfile.

There are currently no tests for as_strategy (an old test for this was removed because it was not particularly useful and was even misleading).

New tests should be written for as_strategy. Two that certainly make sense are:

1. A consistency test that checks that the expected payoff of the original behavior profile matches the expected payoff computed for the mixed strategy that is return by as_strategy.
2. A reference test that uses manual calculations which implement Kuhn's theorem to convert a behavior strategy to a mixed strategy.

[tturocy]

A fortiori we might consider renaming that to as_mixed_strategy_profile or to_mixed_strategy_profile to be more consistent with nomenclature elsewhere.