internal/tdtest: use field name in default error message

Signed-off-by: Marcel van Lohuizen <mpvl@gmail.com>
Change-Id: I20728f8ec7d6f1bb899c227cebd70ee98dd94c02
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/552273
Unity-Result: CUEcueckoo <cueckoo@cuelang.org>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>

internal/tdtest/tdtest.go

@@ -130,7 +130,8 @@ func (t *T) Equal(actual, field any, msgAndArgs ...any) {
 		info, ci := t.getCallInfo()
 		t.updateField(info, ci, actual)
 	case len(msgAndArgs) == 0:
-		t.Errorf("unexpected value:\ngot:  %v;\nwant: %v", actual, field)
+		_, ci := t.getCallInfo()
+		t.Errorf("unexpected value for field %s:\ngot:  %v;\nwant: %v", ci.fieldName, actual, field)
 	default:
 		format := msgAndArgs[0].(string) + ":\ngot:  %v;\nwant: %v"
 		args := append(msgAndArgs[1:], actual, field)