Move OpenCensus bridge integration tests to their own module

[MrAlias]

Part of #2144 and #2121

Use the default SDK to test the OpenCensus bridge. In order to not add the SDK as a dependency to the bridge module and require users to depend on it if they use the bridge these tests are moved to a new dedicated test module.

This removes the use of the oteltest package in favor of the sdk/trace/tracetest package.

[codecov]

Codecov Report

Merging #2165 (feb1f40) into main (89a9489) will decrease coverage by 0.0%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            main   #2165     +/-   ##
=======================================
- Coverage   72.4%   72.4%   -0.1%     
=======================================
  Files        176     176             
  Lines      12091   12091             
=======================================
- Hits        8759    8755      -4     
- Misses      3094    3098      +4     
  Partials     238     238             

Impacted Files	Coverage Δ
bridge/opencensus/bridge.go	0.0% <0.0%> (-100.0%)	⬇️
bridge/opencensus/utils/utils.go	0.0% <0.0%> (-50.0%)	⬇️

[MrAlias]

The code coverage is offset in #2164