Skip backing-store GC test on Windows

No-try: true
Bug: v8:8169
Change-Id: I2ba158d3a2886c18fc4897e5172f32a2211f1a57
Reviewed-on: https://chromium-review.googlesource.com/1221286Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55821}

test/mjsunit/mjsunit.status

@@ -621,6 +621,9 @@
   'math-floor-of-div-nosudiv': [PASS, ['mode == debug', SKIP]],
   'unicodelctest': [PASS, ['mode == debug', SKIP]],
 
+  # BUG(v8:8169)
+  'external-backing-store-gc': [SKIP],
+
   # Setting the timezone and locale with environment variables unavailable
   'icu-date-to-string': [SKIP],
   'icu-date-lord-howe': [SKIP],