Reduce noise and document test case.
Reduce noise and document test case.

This commit introduces a `assert_drop_order_after_poll` helper function
to the test case for this case to reduce repetitive noise and documents
what each function aims to test.

graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust