Revert "Remove CTestTestfile.cmake when BUILD_TESTING is OFF"

Revert commit v3.8.0-rc1~305^2 (Remove CTestTestfile.cmake when BUILD_TESTING
is OFF, 2016-11-14) again.  We reverted it once in commit v3.8.0-rc3~22^2
(Revert "Remove CTestTestfile.cmake when BUILD_TESTING is OFF", 2017-03-06) but
it was accidentally restored by commit v3.11.0-rc1~387^2 (server: add
"ctestInfo" request to get test info, 2017-10-25), perhaps due to conflict
resolution during rebase.

We cannot remove `CTestTestfile.cmake` when testing is off because it breaks
projects that never enable testing but create their own `CTestTestfile.cmake`
manually instead.  Revert the change again and add a test case.

Source/cmLocalGenerator.cxx

@@ -224,14 +224,7 @@ void cmLocalGenerator::TraceDependencies()
 
 void cmLocalGenerator::GenerateTestFiles()
 {
-  std::string file = this->StateSnapshot.GetDirectory().GetCurrentBinary();
-  file += "/";
-  file += "CTestTestfile.cmake";
-
   if (!this->Makefile->IsOn("CMAKE_TESTING_ENABLED")) {
-    if (cmSystemTools::FileExists(file)) {
-      cmSystemTools::RemoveFile(file);
-    }
     return;
   }
 
@@ -240,6 +233,10 @@ void cmLocalGenerator::GenerateTestFiles()
   const std::string& config =
     this->Makefile->GetConfigurations(configurationTypes, false);
 
+  std::string file = this->StateSnapshot.GetDirectory().GetCurrentBinary();
+  file += "/";
+  file += "CTestTestfile.cmake";
+
   cmGeneratedFileStream fout(file.c_str());
   fout.SetCopyIfDifferent(true);
 

Tests/RunCMake/CTest/CTestTestfile.cmake.in

@@ -0,0 +1 @@
+# Created manually

Tests/RunCMake/CTest/NotOn-check.cmake

@@ -0,0 +1,8 @@
+set(f "${RunCMake_TEST_BINARY_DIR}/CTestTestfile.cmake")
+if(NOT EXISTS "${f}")
+  set(RunCMake_TEST_FAILED "File does not exist:\n  ${f}")
+endif()
+file(READ ${f} content)
+if(NOT "${content}" MATCHES "^# Created manually")
+  set(RunCMake_TEST_FAILED "File:\n  ${f}\nhas unexpected content:\n  ${content}")
+endif()

Tests/RunCMake/CTest/NotOn.cmake

@@ -0,0 +1,3 @@
+set(BUILD_TESTING OFF CACHE BOOL "Build the testing tree.")
+include(CTest)
+configure_file(CTestTestfile.cmake.in CTestTestfile.cmake)

Tests/RunCMake/CTest/RunCMakeTest.cmake

@@ -3,3 +3,5 @@ include(RunCMake)
 set(RunCMake_TEST_OPTIONS -DNoProject=1)
 run_cmake(BeforeProject)
 unset(RunCMake_TEST_OPTIONS)
+
+run_cmake(NotOn)