In tests, we set a test-specific HOME, which causes macOS Keychain to look for
the default keychain of a "different" user. If the default keychain does not
exist, all subsequent calls to macOS Keychain will fail. User interaction
(such as entering the password) is required to create a keychain. However,
user interaction is not feasible in automated tests.

This problem is caused by macOS's assumption that a user must have a login
keychain, which is also the user's default keychain, because a user account is
always created with a login keychain by default. Thus, macOS infers the user
according to the HOME env, and does not expect the user to change the HOME env
during normal use.