Deprecate oteltest.Harness for removal

[MrAlias]

This harness is intended to validate the specified functionality of an SDK. It only partially does this as it has not been maintained as the specification evolved. Having a stale testing structure is acceptable in helping this project prevent regressions until we have time to update or replace it, but exporting this for users to rely on is not okay.

This deprecates the harness with the intent to remove it in the next release as it looks like this is the only published project, outside of copies, that uses it. To achieve the prevention of regressions mentioned above, this makes an internal copy of the harness that is expected to last when the original is removed.

Part of #2121

[codecov]

Codecov Report

Merging #2123 (4007fe6) into main (7a624ac) will decrease coverage by 2.0%.
The diff coverage is 0.0%.

@@           Coverage Diff           @@
##            main   #2123     +/-   ##
=======================================
- Coverage   73.6%   71.5%   -2.1%     
=======================================
  Files        168     169      +1     
  Lines       8234    8235      +1     
=======================================
- Hits        6062    5893    -169     
- Misses      1933    2102    +169     
- Partials     239     240      +1     

Impacted Files	Coverage Δ
internal/internaltest/harness.go	0.0% <0.0%> (ø)
oteltest/harness.go	0.0% <0.0%> (-84.7%)	⬇️
...s/otlp/otlptrace/internal/connection/connection.go	13.7% <0.0%> (-2.5%)	⬇️