Add tests for malformed selectors in readers

[MadVikingGod]

This adds tests for unusual ways that users may configure Reader's AggregationSelector and TemporalitySelector.

This fixes two errors:

1. If the ManualReader's AggregationSelector ever returns nil the reader would panic.
2. If a Pipeline's Reader's AggregationSelector returns nil or DefaultAggreation it wouldn't correctly fall back to the appropriate Aggregation for the InstrumentKind.

Resloves #3665

[codecov]

Codecov Report

Merging #4350 (8ee9cd7) into main (c1a644a) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4350     +/-   ##
=======================================
- Coverage   83.4%   83.4%   -0.1%     
=======================================
  Files        184     184             
  Lines      14376   14383      +7     
=======================================
+ Hits       12003   12006      +3     
- Misses      2145    2149      +4     
  Partials     228     228             

Files Changed	Coverage Δ
sdk/metric/reader.go	100.0% <ø> (ø)
sdk/metric/manual_reader.go	76.8% <100.0%> (+0.5%)	⬆️
sdk/metric/pipeline.go	90.7% <100.0%> (-1.1%)	⬇️