[test] Run only default variant for slow test on arm64

TBR=petermarshall@chromium.org
NOTRY=true

Bug: v8:7247
Change-Id: Ib59170290ee49bff88595b10bcdda0045325ce36
Reviewed-on: https://chromium-review.googlesource.com/844082Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50308}

test/mjsunit/mjsunit.status

@@ -346,6 +346,9 @@
   'unicode-test': [PASS, SLOW],
   'wasm/atomics': [PASS, SLOW],
   'whitespaces': [PASS, SLOW],
+
+  # BUG(v8:7247).
+  'regress/regress-779407': [PASS, SLOW, NO_VARIANTS],
 }],  # 'arch == arm64'
 
 ['arch == arm64 and mode == debug and simulator_run', {