Streaming go sample test

[manitgupta]

• Provides a sample test for streaming go with minimally invasive changes to test the LaunchDataflowJob function.
• It does not refactor the whole file, just adds a unit test for the specified public method.
• It mocks out the Dataflow and Datastream client external API calls.
• It can be said with a high degree of confidence that these changes are not breaking and most likely will not cause a regression.

[codecov]

Codecov Report

Merging #752 (a3490ed) into master (e323330) will increase coverage by 0.41%.
Report is 2 commits behind head on master.
The diff coverage is 42.30%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #752      +/-   ##
==========================================
+ Coverage   46.06%   46.47%   +0.41%     
==========================================
  Files         122      122              
  Lines       17120    17123       +3     
  Branches      471      471              
==========================================
+ Hits         7886     7958      +72     
+ Misses       8739     8661      -78     
- Partials      495      504       +9     

Components	Coverage Δ
backend-apis	43.33% <ø> (ø)
backend-library	53.51% <42.30%> (+0.71%)	⬆️
cli	0.00% <ø> (ø)
frontend	32.08% <ø> (+0.11%)	⬆️

Files	Coverage Δ
streaming/streaming.go	18.47% <42.30%> (+9.93%)	⬆️

... and 1 file with indirect coverage changes