make ostree_cmd__private__ ret value const
tests: fix --help test

Check for "Usage" only in the root command, builtins may not output
it.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
tests: test-admin-deploy-switch doesn't use deprecated "current" symlink

commit dfeb27eca55d923c57735e491e438ae54f8cc201 removed it, so change
the test to not use it.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>