partest.MemoryTest based tests are flaky

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see e.g. scala/scala#5480 for further discussion

simply getting rid of MemoryTest-based tests is an option; 5480 does that for one

another option would be to improve MemoryTest as Iulian suggests at scala/scala#5480 (comment) (including a link to similar but superior code we could borrow from ScalaIDE). if that happens, 5480 could be reverted.

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Imported From: https://issues.scala-lang.org/browse/SI-10076?orig=1
Reporter: @SethTisue