tests: Fix mix up of actual/expected in error messages.
tests: c-api: Add a cleanup function for tests.

By specifying a cleanup function we can ensure that Augeas and hivex
functions can be tested.

There is no functional change here, verified by diffing the generated
file tests/c-api/tests.c before and after.
tests: Test result of aug_get properly.

Don't just check aug_get runs, check the resulting string is correct.
tests: Test aug_set.
tests: Test aug_insert, aug_clear and aug_save.
tests: Add test for aug_ls.