PPC: Add whitespaces and grow-memory to slow test

Bug: 
Change-Id: I957e96129d9194eaae406383454f7cd09f53c97f
Reviewed-on: https://chromium-review.googlesource.com/753514Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49145}

test/mjsunit/mjsunit.status

@@ -607,6 +607,8 @@
   'regress/regress-91008': [PASS, SLOW],
   'regress/regress-1257': [PASS, SLOW],
   'harmony/regexp-property-lu-ui': [PASS, SLOW],
+  'whitespaces': [PASS, SLOW],
+  'wasm/grow-memory': [PASS, SLOW],
 }],  # 'arch == ppc and simulator_run'
 
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