logd: add tests for log deduplication

Fix a subtle bug that liblog event messages have a payload of int32_t,
not uint32_t, so they should only be summed to int32_t max.

Make a bunch of test improvements as well to support these.

Test: these tests
Change-Id: I4069cc546240bfffec5b19f34ebec913799674e8
3 files changed