Add expectations about auto-inertial warnings

[scpeters]

Targeting #1513.

This updates expectations for three test cases related to auto-inertial warnings by using the ERR enforcement policy in ParserConfig:

• ResolveAutoInertialsWithDifferentDensity: parse first with enforcement policy ERR and expect a warning about unset collision density. Keep the subsequent parsing with default enforcement policy that expects no errors but will print the warning to the console.
• InertialValuesGivenWithAutoSetToTrue: parse first with enforcement policy ERR and expect a warning about user-specified inertial values. Again keep the subsequent parsing with default enforcement policy that expects no errors but will print the warning to the console.
• ResolveAutoInertialsWithMassAndDefaultDensity: remove the user-specified //inertial/pose and //inertial/inertia values, parse with enforcement policy ERR and expect no warnings/errors.

[iche033]

looks good to me, thanks!