Generate the ART MTS test list file of "eng-only" tests.

Have `regen-test-files` also generate file
`tools/mts-tradefed/res/config/mts-art-tests-list-eng-only.xml`, which
contains the list of tests that need root access to the
device-under-test and are not part of "user" test plans.

Flag: TEST_ONLY
Test: Run `test/utils/regen-test-files -m` and verify it is "almost" a no-op
Bug: 268491069
Bug: 167385698
Change-Id: I68945b458c6a7a29ab5f0ea87cc554b895f062cc