Intermittent failures in PrefetchVerbTests

[jamill]

Test failure:

2018-08-16T00:12:13.9761846Z Test GVFS.FunctionalTests.Tests.EnlistmentPerFixture.PrefetchVerbTests.PrefetchCleansUpStalePrefetchLock()
2018-08-16T00:12:13.9762591Z Failed at 12:12:13 AM taking 00:00:03.8187203
2018-08-16T00:12:13.9763367Z   Expected local cache root to contain 2 items. Actual items: C:\Repos\GVFSFunctionalTests\.gvfsCache\b810a5d8b978423bb9fe3aa140499e0e,C:\Repos\GVFSFunctionalTests\.gvfsCache\mapping.dat,C:\Repos\GVFSFunctionalTests\.gvfsCache\mapping.dat.lock
2018-08-16T00:12:13.9764110Z   Expected: 2
2018-08-16T00:12:13.9764419Z   But was:  3
2018-08-16T00:12:13.9764606Z 

Instances of failing builds:
18227.3

[wilbaker]

Ported to M139 in #194