use std::string::rfind() instead of open coding it

While at it avoid creating a new string.

Source/CTest/cmCTestBuildHandler.cxx

@@ -386,24 +386,20 @@ int cmCTestBuildHandler::ProcessHandler()
   if (this->CTest->GetCTestConfiguration("SourceDirectory").size() > 20) {
     std::string srcdir =
       this->CTest->GetCTestConfiguration("SourceDirectory") + "/";
-    for (cc = srcdir.size() - 2; cc > 0; cc--) {
-      if (srcdir[cc] == '/') {
-        srcdir = srcdir.substr(0, cc + 1);
-        break;
-      }
+    cc = srcdir.rfind('/', srcdir.size() - 2);
+    if (cc != std::string::npos) {
+      srcdir.resize(cc + 1);
+      this->SimplifySourceDir = std::move(srcdir);
     }
-    this->SimplifySourceDir = srcdir;
   }
   if (this->CTest->GetCTestConfiguration("BuildDirectory").size() > 20) {
     std::string bindir =
       this->CTest->GetCTestConfiguration("BuildDirectory") + "/";
-    for (cc = bindir.size() - 2; cc > 0; cc--) {
-      if (bindir[cc] == '/') {
-        bindir = bindir.substr(0, cc + 1);
-        break;
-      }
+    cc = bindir.rfind('/', bindir.size() - 2);
+    if (cc != std::string::npos) {
+      bindir.resize(cc + 1);
+      this->SimplifyBuildDir = std::move(bindir);
     }
-    this->SimplifyBuildDir = bindir;
   }
 
   // Ok, let's do the build