Fix test failure on GPU for parallel lbfgs

[scarrazza]

Fixes test_vqe crash on GPU when running the parallel l-bfgs-b.

[codecov]

Codecov Report

Merging #297 (9eeaf7f) into master (568bb2d) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #297   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           55        55           
  Lines        10583     10583           
=========================================
  Hits         10583     10583           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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Impacted Files	Coverage Δ
src/qibo/tests/test_vqe.py	100.00% <ø> (ø)

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[stavros11]

If we don't want to skip this we can also use with pytest.raises(RuntimeError), but either approach should be fine.