[gax-java] chore: include only external-facing gax testing utils in testlibJar

[miraleung]

This PR fixes #700 by removing unit tests from the testlib and sources jar. On doing ./gradlew testlibJar, I now see only the following files in the resulting {gax,gax-grpc,gax-httpjson}-1.56.0.testlib.jar files:

com/google/api/gax/grpc/testing/**
com/google/api/gax/httpjson/testing/**
com/google/api/gax/rpc/testing/**

[codecov]

Codecov Report

Merging #1120 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1120   +/-   ##
=========================================
  Coverage     78.72%   78.72%           
  Complexity     1169     1169           
=========================================
  Files           204      204           
  Lines          5184     5184           
  Branches        416      416           
=========================================
  Hits           4081     4081           
  Misses          928      928           
  Partials        175      175           

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[vam-google]

How exactly to define the external-facing gax testing utils here? I see this puts only the "testing" subfolders, but is that it? There are other infrastructure classes, which are not in testing subfodlers. For example com.google.api.gax.core.FakeApiClock or com.google.api.gax.rpc.AccumulatingStreamObserver.

Also, does this solve the other part of the issue (missing source files in the artifact)?

[miraleung]

@elharo to confirm whether the external-facing utils are limited to the testing packages.

The testlib sources are already in the sources jar, but I've added something to strip out unit tests as well.

[elharo]

does this fix a bug? If so, add it to the PR description.

[miraleung]

Done, and this PR is already linked to issue #700.