[test_runner] Better name ELF and Mach-O files directly produced by gen_snapshot.

Change-Id: I2afac0207c40daccf9eb236a278cd1fb39c42876 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/456422 Reviewed-by: Alexander Markov <alexmarkov@google.com>

Author: rmacnak-google

pkg/test_runner/lib/src/compiler_configuration.dart

@@ -1423,6 +1423,12 @@ abstract mixin class VMKernelCompilerMixin {
  return Path('$tempDir/out.dll').toNativePath();
  }
  }
+ if (_configuration.genSnapshotFormat == GenSnapshotFormat.machODylib) {
+ return Path('$tempDir/libout.dylib').toNativePath();
+ }
+ if (_configuration.genSnapshotFormat == GenSnapshotFormat.elf) {
+ return Path('$tempDir/libout.so').toNativePath();
+ }
  return Path('$tempDir/out.aotsnapshot').toNativePath();
  }