Deprecate the oteltest package

[MrAlias]

Now that the otel package no longer depends on the oteltest package (#2077), the oteltest package should be refactored similar to #2075.

• Any testing functionality provided needs to reuse as much of the default SDK as possible so testers can be assured when the default SDK is used it will work as expected.
• The unused or unneeded parts of the package should be removed or made internal.

[MrAlias]

Proposal to standardize integration testing for instrumentation libraries: open-telemetry/opentelemetry-go-contrib#897

[MrAlias]

#2165 removes the last use of the oteltest package in this repository. After it merges a PR to document the oteltest package can be created and this issue will be resolved.

[MrAlias]

#2165 removes the last use of the oteltest package in this repository. After it merges a PR to document the oteltest package can be created and this issue will be resolved.

In order to not have a huge burden during upgrade, we should probably purge opentelemetry-go-contrib before deprecating oteltest here.

[MrAlias]

Dependencies in contrib:

$ grep -Rn 'go.opentelemetry.io/otel/oteltest' | grep -v 'go\.\(mod\|sum\)' | grep -v '^\.git'
instrumentation/google.golang.org/grpc/otelgrpc/grpc_test.go:32:	"go.opentelemetry.io/otel/oteltest"
instrumentation/google.golang.org/grpc/otelgrpc/interceptor_test.go:26:	"go.opentelemetry.io/otel/oteltest"
instrumentation/google.golang.org/grpc/otelgrpc/benchmark_test.go:28:	"go.opentelemetry.io/otel/oteltest"
instrumentation/go.mongodb.org/mongo-driver/mongo/otelmongo/mongo_test.go:29:	"go.opentelemetry.io/otel/oteltest"
instrumentation/net/http/otelhttp/handler_test.go:30:	"go.opentelemetry.io/otel/oteltest"
instrumentation/net/http/otelhttp/transport_test.go:25:	"go.opentelemetry.io/otel/oteltest"
instrumentation/net/http/otelhttp/client_test.go:28:	"go.opentelemetry.io/otel/oteltest"
instrumentation/net/http/otelhttp/config_test.go:26:	"go.opentelemetry.io/otel/oteltest"
instrumentation/net/http/httptrace/otelhttptrace/httptrace_test.go:29:	"go.opentelemetry.io/otel/oteltest"
instrumentation/net/http/httptrace/otelhttptrace/clienttrace_test.go:29:	"go.opentelemetry.io/otel/oteltest"
instrumentation/github.com/aws/aws-sdk-go-v2/otelaws/aws_test.go:29:	"go.opentelemetry.io/otel/oteltest"
instrumentation/github.com/labstack/echo/otelecho/echo_test.go:33:	"go.opentelemetry.io/otel/oteltest"
instrumentation/github.com/go-kit/kit/otelkit/endpoint_test.go:28:	"go.opentelemetry.io/otel/oteltest"
instrumentation/github.com/bradfitz/gomemcache/memcache/otelmemcache/gomemcache_test.go:28:	"go.opentelemetry.io/otel/oteltest"
instrumentation/github.com/gin-gonic/gin/otelgin/gintrace_test.go:33:	"go.opentelemetry.io/otel/oteltest"
instrumentation/github.com/gocql/gocql/otelgocql/gocql_test.go:34:	"go.opentelemetry.io/otel/oteltest"
instrumentation/github.com/emicklei/go-restful/otelrestful/restful_test.go:31:	"go.opentelemetry.io/otel/oteltest"
instrumentation/github.com/Shopify/sarama/otelsarama/consumer_test.go:28:	"go.opentelemetry.io/otel/oteltest"
instrumentation/github.com/Shopify/sarama/otelsarama/producer_test.go:31:	"go.opentelemetry.io/otel/oteltest"
instrumentation/github.com/gorilla/mux/otelmux/mux_test.go:33:	"go.opentelemetry.io/otel/oteltest"
instrumentation/github.com/astaxie/beego/otelbeego/beego_test.go:34:	"go.opentelemetry.io/otel/oteltest"
instrumentation/gopkg.in/macaron.v1/otelmacaron/macaron_test.go:30:	"go.opentelemetry.io/otel/oteltest"`

[MrAlias]

Tracking the removal in the contrib repo with this project: https://github.com/open-telemetry/opentelemetry-go-contrib/projects/3

@evantorrie feel free to grab an issue from there!