File monitor unit test failures on Windows

```
UnitTests.Distribution.Client.FileMonitor
sanity check mtimes: FAIL (0.01s)
expected different file mtimes
no monitor cache: OK
corrupt monitor cache: OK
empty monitor: OK
missing file: FAIL
expected: MonitoredFileChanged "dir/a"
but got: MonitoredFileChanged "dir\\a"
change file: FAIL (0.02s)
expected change
file mtime vs content: FAIL (0.02s)
expected change
remove file: FAIL
expected: MonitoredFileChanged "dir/a"
but got: MonitoredFileChanged "dir\\a"
non-existent file: OK
glob matches
no change: OK
add match: FAIL (0.02s)
expected change
remove match: FAIL
expected: MonitoredFileChanged "dir/good-a"
but got: MonitoredFileChanged "dir\\good-a"
change match: FAIL (0.01s)
expected change
add match subdir: FAIL (0.01s)
expected change
remove match subdir: FAIL
expected: MonitoredFileChanged "dir/a/good-a"
but got: MonitoredFileChanged "dir\\a\\good-a"
change match subdir: FAIL (0.01s)
expected change
add non-match: OK (0.01s)
remove non-match: OK
add non-match: OK (0.01s)
remove non-match: OK
invariant sorted 1: OK (0.01s)
invariant sorted 2: OK (0.02s)
value unchanged: OK
value changed: OK
value & file changed: FAIL (0.03s)
expected: MonitoredFileChanged "a"
but got: MonitoredValueChanged 42
value updated: OK
```

Full log: https://ci.appveyor.com/project/grayjay/cabal/build/1.0.46#L2086

EDIT: These tests also need to be enabled in appveyor.yml.