move gtest to newly created support directory

doc/installation.dox

@@ -49,7 +49,7 @@ Continuable is a header-only library with one required header-only dependency:
     erasure wrapper to convert a \ref continuable_base into a \ref continuable.
 
 Additionally GTest is required as optional dependency for the asynchronous
-unit testing macros defined in `continuable/continuable-testing.hpp`
+unit testing macros defined in `continuable/support/gtest.hpp`
 if those are used:
 
   - [google/googletest](https://github.com/google/googletest) is used as

include/continuable/support/gtest.hpp

@@ -28,8 +28,8 @@
   SOFTWARE.
 **/
 
-#ifndef CONTINUABLE_TESTING_HPP_INCLUDED
-#define CONTINUABLE_TESTING_HPP_INCLUDED
+#ifndef CONTINUABLE_GTEST_HPP_INCLUDED
+#define CONTINUABLE_GTEST_HPP_INCLUDED
 
 #include <continuable/detail/other/testing.hpp>
 #include <continuable/detail/utility/traits.hpp>
@@ -169,4 +169,4 @@
 
 /// \}
 
-#endif // CONTINUABLE_TESTING_HPP_INCLUDED
+#endif // CONTINUABLE_GTEST_HPP_INCLUDED

test/unit-test/test-continuable.hpp

@@ -31,8 +31,8 @@
 #include <functional>
 
 #include <continuable/continuable-base.hpp>
-#include <continuable/continuable-testing.hpp>
 #include <continuable/continuable.hpp>
+#include <continuable/support/gtest.hpp>
 #include <string>
 
 using cti::detail::identity;