Don't import testing package in production builds

[dmathieu]

Fixes #2783

This changes the global_test package used in some tests to use global, so ResetForTests can be in the same package, and in a test-only file.
It also moves ResetForTests to a util_test.go file, which will only be available at test time, removes testing from production builds.

[codecov]

Codecov Report

Merging #2786 (3d997eb) into main (46a10bb) will decrease coverage by 0.0%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            main   #2786     +/-   ##
=======================================
- Coverage   75.8%   75.7%   -0.1%     
=======================================
  Files        178     178             
  Lines      11707   11700      -7     
=======================================
- Hits        8875    8868      -7     
  Misses      2607    2607             
  Partials     225     225             

Impacted Files	Coverage Δ
internal/global/state.go	100.0% <ø> (ø)
trace.go	66.6% <0.0%> (-33.4%)	⬇️
exporters/jaeger/jaeger.go	91.1% <0.0%> (+0.8%)	⬆️