When running SAML-related feature and unit tests together (i.e. via
`fail-fast` job in the CI), some stubs set up states that leak across
other tests and causes unexpected failures.

This change replaces all instances of `allow/expect_any_instance_of`
with `allow/expect_next_instance_of`. Additionally, the monkey-patched
`post` method is also removed since it inadvertently overrides the
`post` method in subsequent tests.