clone of https://github.com/google/guava/pull/6730 but with Windows testing on JDK8

[cpovirk]

Fix Files.createTempDir and FileBackedOutputStream under Windows services, a rare use case.

Fixes #6634

Relevant to #2686 in that it shows that we would ideally run our Windows testing under both Java 8 and a newer version....

RELNOTES=io: Fixed Files.createTempDir and FileBackedOutputStream under Windows services, a rare use case.
PiperOrigin-RevId: 568604081

[cpovirk]

The failure is an unrelated problem, so I think this is behaving as hoped on Windows under Java 8.

That failure:

  testServiceStartupTimes(com.google.common.util.concurrent.ServiceManagerTest): expected to be at least: 150

During the course of this testing, I've also seen a failure in a near-identical test:

  testServiceStartupDurations(com.google.common.util.concurrent.ServiceManagerTest): expected to be at least: PT0.15S

Here's the full version of the first failure:

testServiceStartupTimes(com.google.common.util.concurrent.ServiceManagerTest)  Time elapsed: 0.362 sec  <<< FAILURE!
expected to be at least: 150
but was                : 146
	at com.google.common.util.concurrent.ServiceManagerTest.testServiceStartupTimes(ServiceManagerTest.java:131)

I suspect that I've seen it only during these Windows Java 8 tests, so I may set up the test to no longer run in that environment (not that we run CI for Windows Java 8, anyway). (As with the other Windows slowness we've seen, I wouldn't be surprised if the problem were less "Windows" and more "emulation.")