Remove TensorFlow dependency from frontend tests

[Mandrenkov]

Context: Currently, running make test-frontend fails if TensorFlow is not installed.

Description of the Change:

• Surrounded TensorFlow import in test_states_probabilities.py with try block.
• Removed TestBackendImport from test_tf_import.py since it is largely a duplicate of TestFrontendImport.
• Fixed test_incorrect_tf_version unit test in test_tf_import.py in the event where TF is not installed.
• Safeguarded TensorFlow import in test_engine_integration.py and tfbackend/__init__.py.
• Fixed obsolete mock import in test_utils_integration.py.

Benefits:

• TensorFlow is no longer a hard dependency for running the frontend tests.

Possible Drawbacks: None.

Related GitHub Issues: None.

[codecov[bot]]

Codecov Report

Merging #639 (051baa5) into master (243d5d7) will increase coverage by 58.37%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master     #639       +/-   ##
===========================================
+ Coverage   39.89%   98.27%   +58.37%     
===========================================
  Files          77       77               
  Lines        9178     9178               
===========================================
+ Hits         3662     9020     +5358     
+ Misses       5516      158     -5358     

Impacted Files	Coverage Δ
strawberryfields/backends/tfbackend/__init__.py	75.00% <100.00%> (+12.50%)	:arrow_up:
strawberryfields/utils/random_numbers_matrices.py	100.00% <0.00%> (+4.54%)	:arrow_up:
strawberryfields/api/result.py	100.00% <0.00%> (+9.09%)	:arrow_up:
strawberryfields/compilers/xstrict.py	100.00% <0.00%> (+10.00%)	:arrow_up:
strawberryfields/backends/base.py	89.16% <0.00%> (+11.66%)	:arrow_up:
strawberryfields/apps/data/feature.py	98.36% <0.00%> (+24.59%)	:arrow_up:
strawberryfields/parameters.py	100.00% <0.00%> (+25.21%)	:arrow_up:
strawberryfields/program.py	100.00% <0.00%> (+32.64%)	:arrow_up:
strawberryfields/apps/points.py	100.00% <0.00%> (+33.33%)	:arrow_up:
... and 48 more

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[Mandrenkov]

Thank you very much for the reviews and feedback, @josh146, @antalszava, and @thisac! 🚀