[test] Use only default variants for Android tests

Though it works with other variants apparently, we don't want to waste to
many resources for now.

NOTRY=true
TBR=sergiyb@chromium.org

Bug: chromium:866862
Change-Id: I8da2808fec183efa1bf325664ecf33f18a5b02fc
Reviewed-on: https://chromium-review.googlesource.com/1213187Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55728}

infra/testing/builders.pyl

@@ -39,10 +39,10 @@
       'os': 'Android',
     },
     'tests': [
-      {'name': 'benchmarks'},
+      {'name': 'benchmarks', 'variant': 'default'},
       {'name': 'd8testing', 'variant': 'default', 'shards': 2},
-      {'name': 'mozilla'},
-      {'name': 'test262', 'shards': 6},
+      {'name': 'mozilla', 'variant': 'default'},
+      {'name': 'test262_variants', 'variant': 'default', 'shards': 6},
     ],
   },
   ##############################################################################