huge refactoring

internal/pkg/xassert/string_test.go

@@ -0,0 +1,20 @@
+package xassert
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/thecodingmachine/gotenberg/test/internalpkg/xerrortest"
+)
+
+func TestStringOfOne(t *testing.T) {
+	rule := StringOneOf([]string{"foo", "bar", "baz"})
+	// should be OK.
+	rule.with("FOO", "foo")
+	err := rule.validate()
+	assert.Nil(t, err)
+	// should not be OK.
+	rule.with("FOO", "qux")
+	err = rule.validate()
+	xerrortest.AssertError(t, err)
+}