Re-enable skipped Windows UWP tests

[cbracken]

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

[cbracken]

Closing this since our intent is to wind down development on, and remove the UWP embedder in order to focus on the Win32 embedder which, over time, will start to adopt Windows SDK support (once our supported OSes is a subset of those that support doing so).

See: #14967 (comment)

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