Consistent unit test header inclusion order

This patch consistently changes the inclusion order for unit test files
to the following:

 1. The header of the unit under test (using <> includes).
 2. The unit_test.hpp header (using "" includes).
 3. Any additional auxiliary test headers (using "" includes and sorted alphabetically).
 4. Additional system headers needed for the test (using <> includes and sorted alphabetically).
 5. Conditionally included system headers (using <> includes).

Putting the unit under test's header at the very beginning has the
advantage of also testing that the header is self-contained.  It also
makes it very quick to tell what unit is tested in this file.

tests/test_extended_conversions.cpp

@@ -1,6 +1,7 @@
+#include <toml.hpp>
+
 #include "unit_test.hpp"
 
-#include <toml.hpp>
 #include <deque>
 #include <map>