core: Improve realism of map representation in testDiffFn

The diffs created by testDiffFn use the flatmap package directly, rather
than running the diff-generation logic in helper/schema. It turns out that
flatmap itself can generate the k+".#" keys used to indicate the length
of a list, but only helper/schema itself knew how to generate the
corresponding k+".%" keys used for maps.

Rather than modifying the now-deprecated flatmap code directly (and risk
breaking assumptions in shims elsewhere), here we just synthesize the
extra required map element within the testDiffFn implementation, which
then in turn allows the MockProvider diffFn shim to correctly recognize
it as a map and convert it into a real cty.Map value to return.

terraform/context_test.go

@@ -8,6 +8,7 @@ import (
 	"os"
 	"path/filepath"
 	"sort"
+	"strconv"
 	"strings"
 	"testing"
 	"time"
@@ -357,6 +358,19 @@ func testFlatAttrDiffs(k string, i interface{}) map[string]*ResourceAttrDiff {
 		diffs[k] = attrDiff
 	}
 
+	// The legacy flatmap-based diff producing done by helper/schema would
+	// additionally insert a k+".%" key here recording the length of the map,
+	// which is for some reason not also done by flatmap.Flatten. To make our
+	// mock shims helper/schema-compatible, we'll just fake that up here.
+	switch t := i.(type) {
+	case map[string]interface{}:
+		attrDiff := &ResourceAttrDiff{
+			Old: "",
+			New: strconv.Itoa(len(t)),
+		}
+		diffs[k+".%"] = attrDiff
+	}
+
 	return diffs
 }