Add the propagation/propagationtest package

[MrAlias]

Move the propagation related types from the oteltest package into the new propagation/propagationtest package. This encapsulates the testing types and allows us to remove the dependency on the oteltest package from the otel package (related to #2077).

[codecov]

Codecov Report

Merging #2113 (71cd299) into main (4935949) will increase coverage by 0.3%.
The diff coverage is 75.6%.

@@           Coverage Diff           @@
##            main   #2113     +/-   ##
=======================================
+ Coverage   72.8%   73.1%   +0.3%     
=======================================
  Files        166     167      +1     
  Lines       8188    8240     +52     
=======================================
+ Hits        5966    6030     +64     
+ Misses      1992    1977     -15     
- Partials     230     233      +3     

Impacted Files	Coverage Δ
oteltest/text_map_propagator.go	76.2% <ø> (ø)
propagation/propagationtest/text_map_carrier.go	73.0% <73.0%> (ø)
propagation/propagationtest/text_map_propagator.go	80.0% <80.0%> (ø)
sdk/export/metric/metrictest/test.go
sdk/metric/processor/processortest/test.go	89.8% <0.0%> (+<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%)	⬆️

[MrAlias]

It looks like the internal/global package is the only place that actually uses this code. It seems like it would make more sense to just move it into the internal package.