[Gardener] Disable NewTab test due to consistent failures on lacros buidlers

Example failure: https://ci.chromium.org/ui/b/8759469559684085857

Bug: b/313557752
Change-Id: I7de6d98700a71dae702451a3501e423a81ded026
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5177510


Owners-Override: Hamda Mare <hmare@google.com>
Commit-Queue: Mariam Ali <alimariam@google.com>
Reviewed-by: Hamda Mare <hmare@google.com>
Cr-Commit-Position: refs/heads/main@{#1244101}