Add ART MTS eng-only tests to the test plan of ART MTS shard 03.

This is a quick and easy way to run ART MTS eng-only tests as part of
ART test code coverage configurations (so that they can contribute to
the line coverage of the tested code), without requiring the creation
of a new ATP test shard.

Restructuring the MTS generation logic to make it cleaner is planned
as a follow-up.

Flag: TEST_ONLY
Test: m mts && mts-tradefed run commandAndExit mts-art
Test: m mts && mts-tradefed run commandAndExit mts-art-shard-03
Bug: 268491069
Bug: 167385698
Change-Id: I7249b8e3ffe1be411ba08c2e74218a050d570ff8