Use char literal instead of string literal with 1 char

In reality, this is a relatively small performance improvement,
especially with the previous improvements removing lots of superfluous
string handling, but still was measurable.

include/internal/catch_test_case_info.hpp

@@ -102,7 +102,7 @@ namespace Catch {
 
         std::ostringstream oss;
         for( std::set<std::string>::const_iterator it = tags.begin(), itEnd = tags.end(); it != itEnd; ++it ) {
-            oss << "[" << *it << "]";
+            oss << '[' << *it << ']';
             std::string lcaseTag = toLower( *it );
             testCaseInfo.properties = static_cast<TestCaseInfo::SpecialProperties>( testCaseInfo.properties | parseSpecialTag( lcaseTag ) );
             testCaseInfo.lcaseTags.insert( lcaseTag );