[test] Enable mjsunit/es6/block-scoping again.

Enable test after fix.

R=adamk@chromium.org, bmeurer@chromium.org

BUG=v8:5873

Review-Url: https://codereview.chromium.org/2648693003
Cr-Commit-Position: refs/heads/master@{#42550}

test/mjsunit/mjsunit.status

@@ -241,8 +241,6 @@
   #BUG(v8:5683)
   'wasm/import-memory': [SKIP],
 
-  # BUG(v8:5873): Crashes with --ignition-staging under gc_stress
-  'es6/block-scoping': [SKIP],
 }],  # 'gc_stress == True'
 
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