Add more tests about PostOffice and get_components

[dachengx]

What is the problem / what does the code in this PR do

Can you briefly describe how it works?

Can you give a minimal working example (or illustrate with a figure)?

Please include the following if applicable:

• Update the docstring(s)
• Update the documentation
• Tests to check the (new) code is working as desired.
• Does it solve one of the open issues on github?

Please make sure that all automated tests have passed before asking for a review (you can save the PR as a draft otherwise).

[coveralls]

coverage: 90.465% (+0.2%) from 90.283%
when pulling 406e92a on add_more_tests
into ac232cc on master.