Indicate missing attributes in `knownvalue.ObjectExact` failure messages

[gdavison]

terraform-plugin-testing version

github.com/hashicorp/terraform-plugin-testing v1.11.0

Use cases

When assessing a test failure, I want the error message to indicate what has failed

Attempted solutions

Setting a breakpoint in knownvalue.ObjectExact and running the debugger

Proposal

Currently, if the number of expected and actual attributes in knownvalue.ObjectExact don't match, the check immediately exits and returns an error such as

expected 2 attributes for ObjectExact check, got 3 attributes

without indicating what the additional attributes are. This can happen in plan checks, as unknown values are not included at all in the value.

[austinvalle]

Definitely not a great error message, we're about to create a new release so will update this to be more helpful 👍🏻