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.