lang: unification: Allow err string comparisons in tests

Let's improve our test infra to make it more capable. It's important to
catch we failed for the _right_ reason so as to not mask the wrong
errors.

lang/unification_test.go

@@ -21,6 +21,7 @@ package lang
 
 import (
 	"fmt"
+	"strings"
 	"testing"
 
 	"github.com/purpleidea/mgmt/lang/funcs"
@@ -37,6 +38,7 @@ func TestUnification1(t *testing.T) {
 		fail      bool
 		expect    map[interfaces.Expr]*types.Type
 		experr    error  // expected error if fail == true (nil ignores it)
+		experrstr string // expected error prefix
 	}
 	testCases := []test{}
 
@@ -656,7 +658,7 @@ func TestUnification1(t *testing.T) {
 		}
 		names = append(names, tc.name)
 		t.Run(fmt.Sprintf("test #%d (%s)", index, tc.name), func(t *testing.T) {
-			ast, fail, expect, experr := tc.ast, tc.fail, tc.expect, tc.experr
+			ast, fail, expect, experr, experrstr := tc.ast, tc.fail, tc.expect, tc.experr, tc.experrstr
 
 			//str := strings.NewReader(code)
 			//ast, err := LexParse(str)
@@ -735,6 +737,14 @@ func TestUnification1(t *testing.T) {
 			if fail && err != nil {
 				t.Logf("test #%d: err: %+v", index, err)
 			}
+			// test for specific error string!
+			if fail && experrstr != "" && !strings.HasPrefix(err.Error(), experrstr) {
+				t.Errorf("test #%d: FAIL", index)
+				t.Errorf("test #%d: expected fail, got wrong error", index)
+				t.Errorf("test #%d: got error: %s", index, err.Error())
+				t.Errorf("test #%d: exp error: %s", index, experrstr)
+				return
+			}
 
 			if expect == nil { // test done early
 				return