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_find.cpp

@@ -1,15 +1,17 @@
+#include <toml.hpp>
+
 #include "unit_test.hpp"
 
-#include <toml.hpp>
-#include <map>
-#include <unordered_map>
-#include <list>
-#include <deque>
 #include <array>
+#include <deque>
+#include <list>
+#include <map>
+#include <tuple>
+#include <unordered_map>
+
 #if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201703L
 #include <string_view>
 #endif
-#include <tuple>
 
 using test_value_types = std::tuple<
     toml::basic_value<toml::discard_comments>,
@@ -823,4 +825,3 @@ BOOST_AUTO_TEST_CASE_TEMPLATE(test_get_toml_offset_datetime, value_type, test_va
         BOOST_TEST(tm.tm_sec ==            0);
     }
 }
-