In order to enable all the currently-passing Windows UWP unit tests and prevent any regressions, I've disabled currently-failing tests and am enabling these unit tests as part of presubmit.

The failures fall into the following categories:

• threading issues (accessing an object from the wrong thread)
• filesystem sandboxing issues (trying to access a path outside the sandbox)
• network issues (trying to connect to sandboxed vm service)

We should fix and re-enable these. Known tests at time of filing:

• ShellTest.DisallowedDartVMFlag
• ShellTest.AllowedDartVMFlag
• ShellTest.OnPlatformViewDestroyDisablesThreadMerger
• ShellTest.WaitForFirstFrame
• ShellTest.WaitForFirstFrameMultiple
• ShellTest.WaitForFirstFrameInlined
• ShellTest.OnServiceProtocolGetSkSLsWorks
• ShellTest.OnServiceProtocolSetAssetBundlePathWorks
• ShellTest.AssetManagerSingle
• ShellTest.AssetManagerMulti
• PersistentCacheTest.CacheSkSLWorks
• PersistentCacheTest.CanLoadSkSLsFromAsset
• PersistentCacheTest.CanRemoveOldPersistentCache
• PersistentCacheTest.CanPurgePersistentCache
• PersistentCacheTest.PurgeAllowsFutureSkSLCache