Report fork spec failures

[marcotc]

When a test running under our expect_in_fork test helper fails, it creates this output:

  1) Datadog::Workers::AsyncTraceWriter integration tests forking when the process forks and a trace is written with :async fork policy does not drop any traces
     Failure/Error: expect(status && status.success?).to be true
     
       expected true
            got false
     # ./spec/support/synchronization_helpers.rb:13:in `expect_in_fork'
     # ./spec/ddtrace/workers/trace_writer_spec.rb:688:in `block (6 levels) in <top (required)>'
     # ./spec/spec_helper.rb:164:in `block (2 levels) in <top (required)>'(required)>'

This message unfortunately only tells us that the child process exited with a status non-zero.
This error happens to be a flaky issue today: https://app.circleci.com/pipelines/github/DataDog/dd-trace-rb/2998/workflows/5c01a51a-b03d-4bfb-a32d-5015716207df/jobs/120042/parallel-runs/2

This PR captures the STDERR output inside the forked process and reports that output when the fork exists with a non-zero status. I introduced a fake failure (expect(1).to be(2)) at line trace_writer_spec.rb:689 for this example, inside the forked block:

  0) Datadog::Workers::AsyncTraceWriter integration tests forking when the process forks and a trace is written with :async fork policy does not drop any traces
     Failure/Error: expect(status && status.success?).to be(true), stderr

       expected #<Integer:5> => 2
            got #<Integer:3> => 1

       	from ./spec/ddtrace/workers/trace_writer_spec.rb:689:in `block (7 levels) in <top (required)>'
       	from ./spec/support/synchronization_helpers.rb:12:in `block in expect_in_fork'
       	from ./spec/support/synchronization_helpers.rb:8:in `fork'
       	from ./spec/support/synchronization_helpers.rb:8:in `expect_in_fork'
       	from ./spec/ddtrace/workers/trace_writer_spec.rb:688:in `block (6 levels) in <top (required)>'
       	...
       	from /Users/marco.costa/.rbenv/versions/2.7.1/bin/rspec:23:in `<main>'
     # ./spec/support/synchronization_helpers.rb:24:in `expect_in_fork'
     # ./spec/ddtrace/workers/trace_writer_spec.rb:688:in `block (6 levels) in <top (required)>'
     # ./spec/spec_helper.rb:164:in `block (2 levels) in <top (required)>'

[ivoanjo]

Awesome, thanks!