Filter out FileUtilMultiThreadedTest.MultiThreadedTempFiles on Fuchsia.

This test runs too slowly on Fuchsia bots, due to emulation overheads.

Bug: 826408, 844416
Change-Id: Ieada8fc8ca56583a34d8c24f1eecbcc4f6b45852
Reviewed-on: https://chromium-review.googlesource.com/1073731Reviewed-by: Scott Graham <scottmg@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562049}

testing/buildbot/filters/fuchsia.base_unittests.filter

@@ -54,3 +54,6 @@
 
 # Flaky, https://crbug.com/811881
 -AddressSpaceRandomizationTest.RandomBitCorrelations*
+
+# Too slow due to QEMU overheads, https://crbug.com/844416
+-FileUtilMultiThreadedTest.MultiThreadedTempFiles