Remove several metrics test helpers

[jmacd]

Fixes #872.

This removes two test helpers that could be easily replaced by calls into the better-developed test helpers in sdk/metric/processor/processortest. The replacements are equivalent except for a single test that was removed because it was difficult to replace and offered little extra test coverage.

Adds (processortest.Processor).Reset() helper method.

[codecov]

Codecov Report

Merging #2105 (f323a4d) into main (4935949) will increase coverage by 0.2%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #2105     +/-   ##
=======================================
+ Coverage   72.8%   73.1%   +0.2%     
=======================================
  Files        166     165      -1     
  Lines       8188    8158     -30     
=======================================
+ Hits        5966    5968      +2     
+ Misses      1992    1961     -31     
+ Partials     230     229      -1     

Impacted Files	Coverage Δ
sdk/metric/processor/processortest/test.go	89.8% <100.0%> (+<0.1%)	⬆️
exporters/jaeger/jaeger.go	94.2% <0.0%> (+1.1%)	⬆️
exporters/stdout/stdoutmetric/metric.go	80.0% <0.0%> (+1.4%)	⬆️
sdk/metric/aggregator/aggregatortest/test.go	86.8% <0.0%> (+2.0%)	⬆️