[Generate, Test] Split generate test function into beam search, no beam search

[patrickvonplaten]

• Clean the generate testing functions
• Also should fix flaky behaviour of bad_word_tokens test (see [Generate] Add bad words list argument to the generate function #3367 and https://circleci.com/gh/huggingface/transformers/27997?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link)

[codecov-io]

Codecov Report

Merging #3601 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3601      +/-   ##
==========================================
- Coverage   78.06%   78.05%   -0.01%     
==========================================
  Files         100      100              
  Lines       17134    17134              
==========================================
- Hits        13375    13374       -1     
- Misses       3759     3760       +1     

Impacted Files	Coverage Δ
src/transformers/modeling_tf_utils.py	93.28% <0.00%> (-0.17%)	⬇️
src/transformers/modeling_utils.py	92.10% <0.00%> (ø)

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[thomwolf]

LGTM