Set test backend in conftest

[stavros11]

Currently we use the following three lines

original_backend = qibo.get_backend()
qibo.set_backend(backend)
# ... test code here ...
qibo.set_backend(original_backend)

in every test that runs with all different backends. This PR creates a pytest fixture that does this manipulation through the conftest.py so that we don't have to include the above lines in every test and avoid code repetition.

[codecov]

Codecov Report

Merging #407 (c1a4b00) into movetests (8aca616) will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           movetests      #407    +/-   ##
============================================
  Coverage     100.00%   100.00%            
============================================
  Files             79        79            
  Lines          12225     11397   -828     
============================================
- Hits           12225     11397   -828     

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

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/qibo/tests/test_abstract_circuit_qasm.py	100.00% <ø> (ø)
src/qibo/tests/test_backends_matrices.py	100.00% <ø> (ø)
src/qibo/tests/test_cirq.py	100.00% <ø> (ø)
src/qibo/tests/test_core_circuit_noise.py	100.00% <ø> (ø)
src/qibo/tests/test_core_distcircuit.py	100.00% <ø> (ø)
src/qibo/tests/test_core_distcircuit_execution.py	100.00% <ø> (ø)
src/qibo/tests/test_core_distutils.py	100.00% <ø> (ø)
src/qibo/tests/test_core_fusion.py	100.00% <ø> (ø)
src/qibo/tests/test_core_gates_controlled.py	100.00% <ø> (ø)
src/qibo/tests/test_core_gates_density_matrix.py	100.00% <ø> (ø)
... and 24 more

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[scarrazza]

@stavros11 thanks a lot, I was thinking about that when looking at the other pull request.

I have just a comment, I believe there must be a way to drop the backend argument of each test and use a decorator instead. However, this might be slightly complicated, so I believe we can merge this PR as it is.