Add libdm_test to presubmit

Also add libdm_test to the TEST_MAPPING's presubmit of fs_mgr to run the
test on TH whenever changes are applied to the fs_mgr contents.

Test: TH
Bug: none
Change-Id: Icc6adeeacaefbcfeb494f37fc776eb5a886a5183
Signed-off-by: Alessio Balsini <balsini@google.com>