Use async API names in test descriptions and subTests

Mirrored test method names already match their PEP 380 counterparts via
the `_ayf` suffix. The descriptions and subTest labels now use the async
API names (`asend`, `aclose`, `athrow`, `anext`) so it is clear which
operation each test exercises.

Class docstrings on `TestPEP828Operation` and `TestInterestingEdgeCases`
now state that they mirror their `test_yield_from` counterparts.

Author: johnslavik