Test checks for assembled snapshots via the .dll extension triggering even on non-assembled snapshots.

[sstrickl]

There are new test failures on [vm] Support assembly output for Windows..

The tests

vm/dart/build_id_test RuntimeError (expected Pass)

are failing on configurations

vm-aot-asan-win-release-x64

Test log extract:

--- Command "vm" (took 247ms):
set DART_CONFIGURATION=ReleaseASANX64 & set DART_SUPPRESS_WER=1 & set DART_CRASHPAD_HANDLER=C:\b\s\w\ir\out\ReleaseASANX64\crashpad_handler.exe & set TEST_COMPILATION_DIR=C:/b/s/w/ir/out/ReleaseASANX64/generated_compilations/vm-aot-asan-win-release-x64/runtime_tests_vm_dart_build_id_test & out\ReleaseASANX64\dartaotruntime.exe -Dtest_runner.configuration=vm-aot-asan-win-release-x64 --ignore-unrecognized-flags --packages=C:\b\s\w\ir\.dart_tool\package_config.json C:\b\s\w\ir\out\ReleaseASANX64\generated_compilations\vm-aot-asan-win-release-x64\runtime_tests_vm_dart_build_id_test\out.dll

exit code:
255

stderr:
Unhandled exception:
Expect.isNull(actual: <8931c283f0747a39beb923f73d12f7c4>) fails.
#0      Expect._fail (package:expect/expect.dart:887)
#1      Expect.isNull (package:expect/expect.dart:195)
#2      main (file:///C:/b/s/w/ir/runtime/tests/vm/dart/build_id_test.dart:16)
#3      _delayEntrypointInvocation.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:314)
#4      _RawReceivePort._handleMessage (dart:isolate-patch/isolate_patch.dart:193)

--- Re-run this test:
python3 tools/test.py -n vm-aot-asan-win-release-x64 vm/dart/build_id_test

Fix coming in CL 428200.

/cc @rmacnak-google