fix test_exprevaluator in parallel @vincentchabannes

[romainhild]

• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes?
• Have you successfully run the Feel++ testsuite with your changes locally?
• Have you written Doxygen comments in your contribution ?

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The test pass in sequential and in parallel now:

Test project /ssd/hild/builds/develop_rel/testsuite/feelvf
    Start 47: feelpp_test_exprevaluator-np-6
1/2 Test #47: feelpp_test_exprevaluator-np-6 ...   Passed    4.11 sec
    Start 48: feelpp_test_exprevaluator-np-1
2/2 Test #48: feelpp_test_exprevaluator-np-1 ...   Passed    3.91 sec

100% tests passed, 0 tests failed out of 2