Use Catch for our test suite

`make check` is also added to compile and run the tests
Catch is fetched with cmake automatically into the build directory when needed

tests/config.cpp

@@ -0,0 +1,25 @@
+#include "catch.hpp"
+
+#include <config/config.hpp>
+
+TEST_CASE("Config basic")
+{
+  Config::filename = "test.cfg";
+  Config::file_must_exist = false;
+  Config::set("coucou", "bonjour", true);
+  Config::close();
+
+  bool error = false;
+  try
+    {
+      Config::file_must_exist = true;
+      CHECK(Config::get("coucou", "") == "bonjour");
+      CHECK(Config::get("does not exist", "default") == "default");
+      Config::close();
+    }
+  catch (const std::ios::failure& e)
+    {
+      error = true;
+    }
+  CHECK_FALSE(error);
+}