liblp: add test to presubmit

Add liblp_test to TEST_MAPPING to be triggered as TH presubmit test.

Bug: none
Test: liblp_test
Change-Id: If4d60e816e4e1b321603e1a5ed00e21297567d38
Signed-off-by: Alessio Balsini <balsini@google.com>