Add feature-level regression tests across supported toolchain behaviors

[nradakovic]

What

Add regression tests that validate important toolchain feature behavior directly, rather than relying only on broad configuration-level smoke tests.

The goal is to ensure that the new explicit feature model is validated at the behavior level and that regressions are caught early.

Acceptance Criteria (DoD)

• Tests exist for the important supported feature behaviors of the toolchain.
• Each major supported feature is covered by at least one test or validation scenario.
• Tests fail if a required feature is removed, misconfigured, or no longer applied correctly.
• The repository includes feature-level coverage in addition to configuration-level smoke coverage.

How

• Define a test matrix that maps important features to validation scenarios.
• Add or extend example targets and validation cases to exercise behaviors such as:
  • pthread support
  • coverage instrumentation
  • PIC-related behavior
  • sysroot handling
  • dependency file generation
  • warning mode behavior
  • feature-controlled link behavior
• Reuse the example workspace and existing validation scripts where practical.
• Keep tests focused on observable feature behavior instead of broad end-to-end coverage only.

Out of Scope

• Exhaustive compiler correctness testing.
• Broad CI workflow redesign unrelated to feature coverage.
• Consumer documentation updates.