Exclude rundownvalidation from crossgen tests

[jtschuster]

rundownvalidation requires a MethodILToNativeMap rundown event, but that event is emitted only for JIT-compiled methods. A fully composite ReadyToRun framework and test image can legitimately execute with no eligible JIT methods when tiered compilation is disabled, making this assertion depend on incidental runtime JIT fallback behavior.

Exclude this test from crossgen runs so its assembly remains IL and reliably exercises the JIT-only event, while preserving the existing assertion and process-isolated execution.

Validation:

• Reproduced the exact scheduled last-good payload passing and first-bad payload failing on Windows x64 Checked with DOTNET_TieredCompilation=0, --composite, and --inputbubble.
• Isolated the transition to ILCompiler.ReadyToRun.dll from Enable R2R for unresolved static virtual interface calls #131282 using fixed framework inputs and reciprocal compiler DLL swaps.
• Ran the exact first-bad composite framework payload with test crossgen disabled: the test passed with hasMethodILToNativeMap=True, and no IL-CG2 output was created.

Fixes #131662

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Pull request overview

This PR adjusts the rundownvalidation test project configuration so it is excluded from CrossGen2/R2R test passes, keeping the test assembly as IL (and thus reliably JIT-compiled) during those runs.

Changes:

• Disable CrossGen2 compilation for rundownvalidation by setting <CrossGenTest>false</CrossGenTest>.
• Add an inline comment documenting the rationale for excluding the test from crossgen.