ci: fix incremental build to test dependents of test-infra modules

[gnodet]

Summary

• The incremental build script classified modules as "testable" (eligible for -amd dependent expansion) only when they had a src/test directory. Modules with only src/main — like all test-infra/* modules — were treated as "pom-only" and their dependents were never tested.
• This meant changes to test infrastructure (e.g. container version bumps in test-infra/camel-test-infra-infinispan) never triggered integration tests in consuming modules like camel-infinispan.
• Fix: check for src/main instead of src/test. Modules with source code trigger dependent testing; only true pom-only modules (parent, aggregator poms with no source) are excluded from -amd expansion.

Found while investigating #22316, where a container version bump passed CI without running any infinispan ITs.

Test plan

• Verified test-infra/* modules have src/main but no src/test — correctly reclassified as testable
• Verified parent and aggregator poms (e.g. components/camel-infinispan) have no src/main — still correctly classified as pom-only
• Verified the fix doesn't affect modules with both src/main and src/test (standard components)

[github-actions]

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