I need to write a test class for trigger on a standard object. opportunity.

The plan is package the trigger and test class as a free app so users can re-use the component.

As we all know the test coverages are environment dependent.
They are dependent on factors like

(1) Custom Mandatory fields,
(2) Custom Validation rules &
(3) other custom triggers on the object

Are these best practices around creating test class for standard object which can have a good coverage irrespective of the test environment ?

I need to write a test class for trigger on a standard object. Account.

The plan is package the trigger and test class as a free app so users can re-use the component.

As we all know the test coverages are environment dependent.
They are dependent on factors like

(1) Custom Mandatory fields,
(2) Custom Validation rules &
(3) other custom triggers on the object

Are these best practices around creating test class for standard object which can have a good coverage irrespective of the test environment ?