Improve finalization of formula composition tests

Use the `MockMicrogrid` as a context manager and an async exit stack so
all used components get automatically finalized. Before some
finalization was missing in the tests.

This commit can be reviewed much more easily hiding differences in
spaces (with `diff -w`, or appending `?w=1` to the GitHub URL).

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax