Added os:exec builtin (#2)

Co-authored-by: Leon Mika <lmika@lmika.com>
Reviewed-on: #2

ucl/builtins/os_test.go

@@ -2,8 +2,10 @@ package builtins_test
 
 import (
 	"context"
-	"github.com/stretchr/testify/assert"
 	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
 	"ucl.lmika.dev/ucl"
 	"ucl.lmika.dev/ucl/builtins"
 )
@@ -34,3 +36,25 @@ func TestOS_Env(t *testing.T) {
 		})
 	}
 }
+
+func TestOS_Exec(t *testing.T) {
+	tests := []struct {
+		descr string
+		eval  string
+		want  any
+	}{
+		{descr: "run command 1", eval: `os:exec "echo" "hello, world"`, want: "hello, world\n"},
+		{descr: "run command 1", eval: `os:exec "date" "+%Y%m%d"`, want: time.Now().Format("20060102") + "\n"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.descr, func(t *testing.T) {
+			inst := ucl.New(
+				ucl.WithModule(builtins.OS()),
+			)
+			res, err := inst.EvalString(context.Background(), tt.eval)
+			assert.NoError(t, err)
+			assert.Equal(t, tt.want, res)
+		})
+	}
+}