Factor redundant test boilerplate out into unit_test.hpp helper

tests/unit_test.hpp

@@ -0,0 +1,23 @@
+#ifndef BOOST_TEST_MODULE
+#    error "Please #define BOOST_TEST_MODULE before you #include <unit_test.hpp>"
+#endif
+
+#ifdef UNITTEST_FRAMEWORK_LIBRARY_EXIST
+#    include <boost/test/unit_test.hpp>
+#else
+#    include <boost/test/included/unit_test.hpp>
+#endif
+
+#include <cstdlib>
+#include <string>
+
+static inline auto testinput(const std::string& basename) -> std::string
+{
+    const auto this_or_that = [](const char *const s, const char *const t) { return s ? s : t; };
+    std::string directory = this_or_that(std::getenv("TOMLDIR"), "toml");
+    if (!directory.empty() && directory.back() != '/')
+    {
+        directory.push_back('/');
+    }
+    return directory.append("tests/").append(basename);
+}