move trace_util test helpers behind a feature flag

[ekump]

What does this PR do?

1. Moves test helper function for trace-utils behind a feature-flag called test-utils.
2. Move poll_for_mock_hit and create_send_data functions into test-utils.

To limit trace_test_utils to only being used in tests your Cargo.toml file should look like:

[dependencies]
datadog-trace-utils = { path = "../trace-utils" }

[dev-dependencies]
datadog-trace-utils = { path = "../trace-utils", features = ["test-utils"] }

Motivation

Test code shouldn't be available for use outside of tests.

Because the test-utils need to be used in tests both within trace-utils and outside of trace-utils it limited possible solutions:

• Simply wrapping trace_test_utils with #[cfg(test)] at the module export level doesn't work since it prevents these functions from being imported in tests in other crates.
• Creating a separate "datadog_test_utils" crate creates errors in tests inside trace-utils when the helper functions return objects defined in trace-utils. For example, the Rust compiler thinks there is a type mismatch between datadog_test_utils::send_data::SendData (returned by the create_send_data function) and the SendData type in the send_data.rs unit tests even though they are the same type. No matter how I defined imports, I couldn't get this type mismatch to go away.

How to test the change?

Tests that use these helpers still pass.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.45455% with 7 lines in your changes missing coverage. Please review.

Project coverage is 68.91%. Comparing base (5b9ba35) to head (615233d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #461      +/-   ##
==========================================
- Coverage   68.93%   68.91%   -0.03%     
==========================================
  Files         193      193              
  Lines       25733    25732       -1     
==========================================
- Hits        17739    17732       -7     
- Misses       7994     8000       +6     

Components	Coverage Δ
crashtracker	19.34% <ø> (ø)
datadog-alloc	98.76% <ø> (ø)
data-pipeline	51.45% <ø> (ø)
data-pipeline-ffi	0.00% <ø> (ø)
ddcommon	85.89% <ø> (ø)
ddcommon-ffi	74.15% <ø> (ø)
ddtelemetry	56.09% <ø> (ø)
ipc	81.69% <ø> (ø)
profiling	77.97% <ø> (ø)
profiling-ffi	60.00% <ø> (ø)
serverless	0.00% <ø> (ø)
sidecar	36.49% <100.00%> (-0.61%)	⬇️
sidecar-ffi	0.00% <ø> (ø)
spawn-worker	54.98% <ø> (ø)
trace-mini-agent	69.33% <ø> (ø)
trace-normalization	97.79% <ø> (ø)
trace-obfuscation	95.74% <100.00%> (ø)
trace-protobuf	33.33% <ø> (ø)
trace-utils	90.94% <95.20%> (-0.24%)	⬇️