[BUILD] move client::nosend under test_common

[esigo]

Fixes #1226 (issue)

Changes

As client::nosend is meant to be used only for unit tests, this PR moves it under test_common. We could use this directory for any other common code needed for testing.

This directory is compiled only when BUILD_TESTING is set to ON.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #1811 (ac148a8) into main (6a99fee) will decrease coverage by 0.02%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1811      +/-   ##
==========================================
- Coverage   85.79%   85.77%   -0.01%     
==========================================
  Files         171      171              
  Lines        5240     5240              
==========================================
- Hits         4495     4494       -1     
- Misses        745      746       +1     

Impacted Files	Coverage Δ
sdk/src/trace/batch_span_processor.cc	90.63% <0.00%> (-0.78%)	⬇️

[owent]

LGTM

[lalitb]

How about moving this to exporters/test_common or ext/test_common, just to keep the root of repo clean, with only required directories ?