Remove some hard-coded np.linalg calls from tests

[mlazzarin]

In this PR and in qiboteam/qibojit#36 I am trying to fix issue qiboteam/qibojit#34 concerning tests with AMD GPUs.

In particular, there are two tests in Qibo with calls to np.linalg.eigh and np.linalg.eigvalsh.
When these calls are performed with a CuPy object as argument, it seems that somehow there is a automatic call to cp.linalg.eigh or cp.linalg.eigvalsh (I think that this behavior is very confusing).
The problem is that this behavior bypasses the fallback mechanism implemented in qiboteam/qibojit#36 and a rocblas_not_implemented_status error is raised.

The easiest fix is to replace the hard-coded call to NumPy with the backend K in these tests.
However, I wonder if there was a specific reason to use NumPy in these tests, e.g. to compare the results with K.
@scarrazza @stavros11 what do you think?

[stavros11]

Thanks for this update. I do not see any particular reason for using numpy explicitly here, probably using K is a better choice. It may be a "leftover" from before we implemented the backend mechanisms in qibo.

Also, I am not sure if you already looked on this but the same modification should fix #502.

[codecov]

Codecov Report

Merging #501 (2dba502) into rmbackends (f5f0e9f) will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           rmbackends     #501   +/-   ##
===========================================
  Coverage       99.99%   99.99%           
===========================================
  Files              84       84           
  Lines           11853    11853           
===========================================
  Hits            11852    11852           
  Misses              1        1           

Flag	Coverage Δ
unittests	99.99% <100.00%> (ø)

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

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