Add test for Validate crash

Crash during Validate walk with nested variable default.

config/config_test.go

@@ -201,6 +201,12 @@ func TestConfigValidate_table(t *testing.T) {
 			true,
 			"cannot contain interp",
 		},
+		{
+			"nested types in variable default",
+			"validate-var-nested",
+			true,
+			"ERROR",
+		},
 	}
 
 	for i, tc := range cases {

config/test-fixtures/validate-var-nested/main.tf

@@ -0,0 +1,6 @@
+variable "foo" {
+  default = [["foo", "bar"]]
+}
+variable "bar" {
+  default = [{foo = "bar"}]
+}