core: testProviderSchema aws_instance must include "unknown" attribute

This is computed in the special case where compute = "unknown" in order
to force inclusion of an unknown value into the ultimate result, which
is invalid.

This fixes TestContext2Apply_unknownAttribute, which is intending to test
this error handling behavior.

terraform/context_apply_test.go

@@ -8281,7 +8281,7 @@ func TestContext2Apply_unknownAttribute(t *testing.T) {
 
 	state, diags := ctx.Apply()
 	if !diags.HasErrors() {
-		t.Fatal("should error with UnknownVariableValue")
+		t.Error("should error, because attribute 'unknown' is still unknown after apply")
 	}
 
 	actual := strings.TrimSpace(state.String())

terraform/context_test.go

@@ -531,6 +531,12 @@ func testProviderSchema(name string) *ProviderSchema {
 						Type:     cty.String,
 						Computed: true,
 					},
+
+					// Generated by testDiffFn if compute = "unknown" is set in the test config
+					"unknown": {
+						Type:     cty.String,
+						Computed: true,
+					},
 				},
 			},
 			name + "_eip": {