Flaky failures of standalone/file_test

[rmacnak-google]

standalone/file_test flaky fails on several configurations with

Unhandled exception:
PathNotFoundException: Cannot retrieve access time, path = 'out/DebugX64/dartaotruntime' (OS Error: No such file or directory, errno = 2)
#0      _checkForErrorResponse (dart:io/common.dart:58)
#1      _File.lastAccessed.<anonymous closure> (dart:io/file_impl.dart:469)
<asynchronous suspension>
#2      FileTest.testLastAccessed.<anonymous closure> (file:///b/s/w/ir/tests/standalone/io/file_test.dart:1381)
<asynchronous suspension>

log

This test has a large number of subtests that are run without awaiting the previous subtest before starting next subtest, so this likely is caused by testLastAccessed getting interleaved with testAbsolute, which mutates the process-global CWD.