[lldb/test] Disable NSDate format check under _WIN32

Disable the test which attempts to format an NSDate with a date_value of
0 on _WIN32.

When _WIN32 is defined, GetOSXEpoch returns a date that should be in
2001, but after this is passed through timegm (which, afaict isn't
portable?) the result is a date in 1970:

```
lldb-x64-windows-ninja\llvm-project\lldb\unittests\DataFormatter\MockTests.cpp(39): error:       Expected: *formatDateValue(0)
      Which is: "1970-01-01 00:00:00 Pacific Standard Time"
      To be equal to: "2001-01-01 00:00:00 UTC"
```

http://lab.llvm.org:8011/builders/lldb-aarch64-ubuntu/builds/4520/steps/test/logs/stdio