Surface `run_tests.py` diagnostics in `alloy.py` build logs when tests fail

[aneshlya]

When a test job launched from alloy.py fails, the build log only contains a high-level summary (the STABILITY REPORT) followed by a diff of fail_db.txt listing the newly failing tests. For example:

  _________________________STABILITY REPORT_________________________
  Running native - AVX2

  Using test compiler: Intel(r) Implicit SPMD Program Compiler (Intel(r) ISPC), 1.31.0dev (build commit 2857de55446851ee @ 20260622, LLVM 21.1.8)
  Using C/C++ compiler: Microsoft (R) C/C++ Optimizing Compiler Version 19.44.35228 for x64

  x86-64  generic-i32x4 unspec Windows LLVM 21.1         cl O2 obj. Fails: 12, New fails: 12. 00h09m44sec.
  NO NEW RUNFAILS
  NEW COMPFAILS: 12
  NO NEW PASSES RUNFAILS
  NO NEW PASSES COMPFAILS

The summary tells you which tests failed, but not why. The actual diagnostics from run_tests.py (compiler errors, mismatched output, stderr, etc.) are not included in the log, so there is no straightforward way to understand the root cause from the failed job alone.

Problem:

Finding the real diagnostics is non-obvious and time-consuming. To investigate a failure you need to dig through artifacts manually, even though run_tests.py already produced the detailed output during the run.

Suggested improvement:

Make the per-test diagnostics from run_tests.py easily discoverable from the failed alloy.py job — for example, by including the failing tests' compile/run output in the log (or as a saved artifact) alongside the stability report.