[test] Remove obsolete blink test skips

The tests were skipped due to problems with the previous test setup. Now the
setup is the same as in Chromium and those tests should pass.

NOTRY=true

Bug: chromium:828847
Change-Id: Ibfbb931031176add90a340ca79c71e89f05e3045
Reviewed-on: https://chromium-review.googlesource.com/c/1421318Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58918}

tools/blink_tests/TestExpectations

-[ Linux ] virtual/pointerevent/fast/events/mouse-cursor-style-change-iframe.html [ Skip ]
-
-# Turn off Slimming Paint tests on linux.
-[ Linux ] virtual/slimmingpaint/ [ Skip ]
-
-# Several failures since https://crrev.com/c/1196547
-crbug.com/879604 external/wpt/cookies/samesite/ [ Skip ]