test_slapproxy: make the "initial format" behave like it should
Also renames it so it reflects well its function. Before the fix in 77c4cca0, each call to /loadComputerConfigurationFromXML was deleting all the partition in the database. After the fix, because the test environment is not cleaned correctly between different tests, some slapparts would remain between following tests. Thanks to this commit, the previous behavior is restaured.