[eigen] tests taking forever when run with -j8 and OpenMP is enabled

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No, I'm not reporting about a bug here!

But I just stumbled across something interesting. With OpenMP enabled,
when I run

   ctest -j8

that is, 8 tests run simultaneously, the tests using large matrix
product take above of 2 minutes each to complete.

When I run only 1 test at a time (which is the default for ctest),
they complete very quickly.

Of course the reason is that running 8 tests at a time on my 4-core
Core i7 is a recipe for making them fight for the CPU's cache memory.

This is an illustration that setting cache sizes is ultimately
something that only the user can do; the defaults we're using can only
rely on rather unsafe assumptions about competing processes.


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