Tests/*Only: Update cmake_minimum_required versions

Use 3.3 or 2.8.12 where possible.

Tests/COnly/CMakeLists.txt

 # a simple C only test case
-cmake_minimum_required (VERSION 2.6)
+cmake_minimum_required(VERSION 2.8.12)
 project (COnly C)
 
 set(CMAKE_DEBUG_POSTFIX "_test_debug_postfix")

Tests/CSharpOnly/CMakeLists.txt

+cmake_minimum_required(VERSION 3.3)
 # a simple CSharp only test case
 project (CSharpOnly CSharp)
 

Tests/CxxOnly/CMakeLists.txt

 # a simple CXX only test case
+cmake_minimum_required(VERSION 2.8.12)
 project (CxxOnly CXX)
 
 set(CMAKE_DEBUG_POSTFIX "_test_debug_postfix")

Tests/FortranOnly/CMakeLists.txt

-cmake_minimum_required (VERSION 2.8)
+cmake_minimum_required(VERSION 2.8.12)
 project(FortranOnly Fortran)
 message("CTEST_FULL_OUTPUT ")