Simplify posterior shrinkage tests with pytest marks.

[janfb]

there is some code repetition in these tests

https://github.com/sbi-dev/sbi/blob/bb35def3341a0896a66b7038d9c02931f6797113/tests/metrics_test.py#L133-L172

which can be reduced by using @pytest.mark.parametrize (see other tests for examples.).