refactor(otelhttp): unit tests

[knullism]

This is just a small refactoring for otelhttp transport's unit tests to make them do the same way to create wrappedBody instance.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: 5ylar (f5acb90, e27afb9)
• ✅ login: pellared / name: Robert Pająk (76bfd1d)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.8%. Comparing base (ac19587) to head (76bfd1d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #6151   +/-   ##
=====================================
  Coverage   66.8%   66.8%           
=====================================
  Files        206     206           
  Lines      13207   13207           
=====================================
+ Hits        8827    8829    +2     
+ Misses      4112    4111    -1     
+ Partials     268     267    -1     

Files with missing lines	Coverage Δ
instrumentation/net/http/otelhttp/transport.go	95.2% <100.0%> (ø)

... and 1 file with indirect coverage changes

[knullism]

Please check it out. It needs approval to run all of this.

[dmathieu]

I approved to run them earlier, but since you merged your branch with main, the approval was required again.

[pellared]

@5ylar, thank you for your contribution 🏅