ltp: mandatory tests

Re-enable mandatory test feature (unskippable tests).
Some tests need to be made skippable to pass.

Bug: 229291805
Test: atest vts_ltp_test_x86_64
Change-Id: Ibfce0990b510b871993d675366e3c056cc05f935
Signed-off-by: Edward Liaw <edliaw@google.com>
3 files changed