test: add integration test by Bats

[ninoseki]

Add a scaffold for the integration test by using Bats.

I initially tried to use pure Bash for this, but I found that running tests separately, reporting each test result, etc. are difficult.
So I selected Bats as an integration test framework since it's widely used in the wild & its DSL is easy to learn.

It works like the following:

$ bats test          
quotas.bats
 ✓ quotas
search.bats
 ✓ search with limit
 ✓ search with limit & size
 ✓ search with all
user.bats
 ✓ user
version.bats
 ✓ version

6 tests, 0 failures

Notes

• This PR is a scaffolding and I don't implement tests for every command & don't use it in the CI workflow right now

[fw42]

Works for me. Lets try it.

[fw42]

Can you be more specific? Like give an exact command that installs everything you need to run bats?

[ninoseki]

Done by 6dd91a0.