EigenTesting.cmake: Quote argument to separate_arguments.

Eric A. Borischrequested to merge
eborisch/eigen:patch-1 into master

EigenTesting.cmake: Quote argument to separate_arguments.

As described in https://cmake.org/cmake/help/latest/command/separate_arguments.html: "The entire command line must be passed as one string in the argument ."

Reference issue

Fixes #3005 (closed)

Likely also would have fixed #2866 (closed)

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