Show diffs for all previews

[thomas11]

Currently, a failing preview in a test just shows as something like

azure-native:storage:Blob myBlobFile create replacement [diff: ~source]

which is not as helpful as it could be diagnose the problem because you don't know what the diff is.

We could expose a flag for an optional diff in optproviderupgrade but since this is a testing library, I suggest we skip this boilerplate and just turn the diff always on - when would you not want to see the diff on test failure?

With this change:

            +-azure-native:storage:Blob: (replace)
                [id=/subscriptions/0282681f-7a9e-424b-80b2-96babd57a8a1/resourceGroups/rg1c1afbc6/providers/Microsoft.Storage/storageAccounts/store16dfec76/blobServices/default/containers/assets/blobs/mon-fichier.txt]
                [urn=urn:pulumi:test::upgrade-storage-blob::azure-native:storage:Blob::myBlobFile]
              - source: [secret]
              + source: asset(text:678e7ad) {
                    <contents elided>
                }

[danielrbradley]

Approved pending removal of user settings.

[pulumi-bot]

This PR has been shipped in release v0.1.5.