Unmark values in count before go conversion

[pselle]

When evaluating count values, we need to unmark the cty value before passing the value for conversion into a go int value.

This PR adds a test file/some coverage for the evaluateCountExpression function, but selfishly limits this addition to the basic level plus adding a test case specific to this marking.

[codecov]

Codecov Report

Merging #27238 (0d586fd) into master (30126ba) will increase coverage by 0.01%.
The diff coverage is 100.00%.

Impacted Files	Coverage Δ
terraform/eval_count.go	68.42% <100.00%> (+0.56%)	⬆️
dag/marshal.go	61.90% <0.00%> (-1.59%)	⬇️
terraform/node_resource_apply_instance.go	55.46% <0.00%> (+2.52%)	⬆️

[ghost]

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