[test] Bump shards on slow bot

TBR=sergiyb@chromium.org
NOTRY=true

Change-Id: Ia49bf1d4a096e72c6584297ef5db0eaf85938178
Reviewed-on: https://chromium-review.googlesource.com/c/1360573Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58007}

infra/testing/builders.pyl

@@ -896,7 +896,7 @@
   },
   'V8 Linux64 TSAN - isolates': {
     'tests': [
-      {'name': 'v8testing', 'test_args': ['--isolates'], 'shards': 5},
+      {'name': 'v8testing', 'test_args': ['--isolates'], 'shards': 7},
     ],
   },
   'V8 Linux64 UBSan': {