Mark text-fragment redirect test slow

This is a big "many-cases-in-one" test that takes several seconds to
run. It appears to occasionally timeout when run as part of web tests.
This adds the test to SlowTests.

Bug: 1133836
Change-Id: I5a6408ac69228f50e35eb6ec63bd84bc95554dd2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2443542Reviewed-by: Lan Wei <lanwei@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813062}

third_party/blink/web_tests/SlowTests

@@ -538,3 +538,5 @@ crbug.com/1093853 external/wpt/dom/ranges/Range-surroundContents.html [ Slow ]
 
 crbug.com/1104091 [ Linux ] webcodecs/videoframe-imagebitmap.html [ Slow ]
 crbug.com/1093478 external/wpt/quirks/unitless-length/limited-quirks.html [ Slow ]
+
+crbug.com/1133836 external/wpt/scroll-to-text-fragment/redirects.html [ Slow ]