Disable dex2oat test TestWatchdogTrigger with Valgrind.

This test sometimes fails an assertion with Valgrind:

  art/dex2oat/dex2oat_test.cc:137: Failure
  Value of: odex_file.get() == nullptr
    Actual: false
  Expected: true

Disable it while we investigate.

Test: art/test/testrunner/run_build_test_target.py -j8 art-gtest-valgrind64
Bug: 63052624
Change-Id: I75fdbad6f78377b3f80cd2447fb20fc5792cbdcf