Test suite: add tests for lack of output when loading core only

[p-datadog]

What does this PR do?

Motivation:

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^{Visited at: 2025-11-26 02:03:20 UTC}

[datadog-official]

⚠️ Tests

⚠️ Warnings

🧪 1 Test failed

load core only and configure library with no settings produces no output from rspec (Datadog)

Failure/Error: raise("Test script produced unexpected output: #{out}") unless out.empty?

RuntimeError:
./spec/support/loading_helpers.rb:6:in \`run_ruby_code_and_verify_no_output'
./spec/loading_spec.rb:95:in \`block (2 levels) in <top (required)>'
./spec/spec_helper.rb:276:in \`block (2 levels) in <top (required)>'
./spec/spec_helper.rb:156:in \`block (2 levels) in <top (required)>'
/usr/local/bundle/gems/webmock-3.13.0/lib/webmock/rspec.rb:37:in \`block (2 levels) in <top (required)>'
/usr/local/bundle/gems/rspec-wait-0.0.9/lib/rspec/wait.rb:46:in \`block (2 levels) in <top (required)>'
./spec/support/execute_in_fork.rb:32:in \`run'

ℹ️ Info

❄️ No new flaky tests detected

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 7ada20a | Docs | Datadog PR Page | Was this helpful? Give us feedback!}

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-11-26 02:33:02

Comparing candidate commit 7ada20a in PR branch loading-output with baseline commit f045f9d in branch master.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 44 metrics, 2 unstable metrics.