[EPIC] Cross-testing coverage for Update

[iwahbe]

Pursuant to #2773, we are introducing integration level cross-tests (SDKv2, PF) for bridged provider operations. These allow us to easily verify that the behavior of bridged TF providers with Pulumi match the behavior of native terraform providers with Terraform. crosstests are written on a verb by verb basis. This EPIC describes the tests we need for the crosstest.Update verb.

As a pre-requisite to implementing any of these tests, we need:

• Implement crosstest.Update (SDKv2) #2847
• Implement crosstest.Update (PF) #2846

Some of these tests need:

• Cross-test unknown values #1858
• Assert on outputs of crosstest.Update (SDKv2) (similar to Assert on outputs in SDKv2 Cross-tests [Create][SDKv2] #2523)
• Assert on outputs of crosstest.Update (PF) (similar to Assert on outputs in SDKv2 Cross-tests [Create][PF] #2524)

We won't attempt to be as through with testing inputs as we were for #2537, since we presume that inputs and outputs will flow appropriately.

[VenelinMartinov]

I wonder if we can somehow re-use the tests for Diff here - the setup should be similar.