Use StringBuilder::appendLiteral() / StringBuilder::append(char) when possible

Source/web/tests/LocaleICUTest.cpp

Index: Source/web/tests/LocaleICUTest.cpp
diff --git a/Source/web/tests/LocaleICUTest.cpp b/Source/web/tests/LocaleICUTest.cpp
index 5824d19da05a98195997a149cf0774d8ba27c609..c3bbb7564e72ccf1c64fad82bddc6bec27487315 100644
--- a/Source/web/tests/LocaleICUTest.cpp
+++ b/Source/web/tests/LocaleICUTest.cpp
@@ -61,10 +61,10 @@ public:
         String toString() const
         {
             StringBuilder builder;
-            builder.append("labels(");
+            builder.appendLiteral("labels(");
             for (unsigned index = 0; index < m_labels.size(); ++index) {
                 if (index)
-                    builder.append(", ");
+                    builder.appendLiteral(", ");
                 builder.append('"');
                 builder.append(m_labels[index]);
                 builder.append('"');