Fix environment variable mock in test

[TonyCTHsu]

What does this PR do?

Environment variable should be returning a String, this was hack to make our rails test suite functional.

This PR tried to cleanup some of them, but there are some left to be tackled once we figured out how to properly test Rails.

[codecov-commenter]

Codecov Report

Merging #2942 (73be77a) into master (8d3ef4f) will decrease coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2942      +/-   ##
==========================================
- Coverage   98.08%   98.07%   -0.01%     
==========================================
  Files        1301     1301              
  Lines       72065    72057       -8     
  Branches     3310     3306       -4     
==========================================
- Hits        70683    70669      -14     
- Misses       1382     1388       +6     

Impacted Files	Coverage Δ
...ec/datadog/tracing/contrib/rails/support/rails3.rb	94.68% <ø> (-0.06%)	:arrow_down:
...ec/datadog/tracing/contrib/rails/support/rails4.rb	93.82% <ø> (-0.08%)	:arrow_down:
...ec/datadog/tracing/contrib/rails/support/rails5.rb	93.58% <ø> (-0.09%)	:arrow_down:
...ec/datadog/tracing/contrib/rails/support/rails6.rb	94.89% <ø> (-0.06%)	:arrow_down:
...dog/tracing/contrib/rails/rails_active_job_spec.rb	94.73% <100.00%> (ø)
...ing/contrib/rails/rails_log_auto_injection_spec.rb	98.99% <100.00%> (ø)
...rails/rails_semantic_logger_auto_injection_spec.rb	96.55% <100.00%> (ø)
spec/datadog/tracing/contrib/rails/support/base.rb	80.85% <100.00%> (-0.79%)	:arrow_down:

... and 2 files with indirect coverage changes

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