🆕 [ut] BDD/Spec syntax support

Problem:
- `BDD` is available out of the box, however it should be opt in.
- `Spec` syntax is not supported.

Solution:
- Move `given/when/then` to `bdd` namespace and make it opt in.
- Add `describe/it` to `spec` namespace and make it opt in.

example/BDD.cpp

@@ -9,6 +9,7 @@
 
 int main() {
   using namespace boost::ut;
+  using namespace boost::ut::bdd;
 
   "Scenario"_test = [] {
     given("I have...") = [] {

example/CMakeLists.txt

@@ -63,6 +63,7 @@ example(run_report run_report)
 example(section section)
 example(should should)
 example(skip skip)
+example(spec spec)
 example(suite suite)
 example(test _test)
 example(tmp tmp)

example/spec.cpp

@@ -0,0 +1,21 @@
+//
+// Copyright (c) 2019-2020 Kris Jusiak (kris at jusiak dot net)
+//
+// Distributed under the Boost Software License, Version 1.0.
+// (See accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+//
+#include <boost/ut.hpp>
+
+constexpr auto sum = [](auto... args) { return (0 + ... + args); };
+
+int main() {
+  using namespace boost::ut;
+  using namespace boost::ut::spec;
+
+  describe("sum") = [] {
+    it("should be 0") = [] { expect(sum() == 0_i); };
+
+    it("should sum args...") = [] { expect(sum(1, 2, 3) == 6_i); };
+  };
+}

include/boost/ut.hpp

@@ -1987,15 +1987,6 @@ constexpr auto operator""_b(const char*, decltype(sizeof(""))) {
 };
 [[maybe_unused]] constexpr auto should = test;
 [[maybe_unused]] constexpr auto skip = detail::skip{};
-[[maybe_unused]] constexpr auto given = [](const auto name) {
-  return detail::test{"given", name};
-};
-[[maybe_unused]] constexpr auto when = [](const auto name) {
-  return detail::test{"when", name};
-};
-[[maybe_unused]] constexpr auto then = [](const auto name) {
-  return detail::test{"then", name};
-};
 template <class T = void>
 [[maybe_unused]] constexpr auto type = detail::type_<T>();
 
@@ -2029,6 +2020,27 @@ template <class T>
   return const_cast<T&>(t);
 }
 
+namespace bdd {
+[[maybe_unused]] constexpr auto given = [](const auto name) {
+  return detail::test{"given", name};
+};
+[[maybe_unused]] constexpr auto when = [](const auto name) {
+  return detail::test{"when", name};
+};
+[[maybe_unused]] constexpr auto then = [](const auto name) {
+  return detail::test{"then", name};
+};
+}  // namespace bdd
+
+namespace spec {
+[[maybe_unused]] constexpr auto describe = [](const auto name) {
+  return detail::test{"describe", name};
+};
+[[maybe_unused]] constexpr auto it = [](const auto name) {
+  return detail::test{"it", name};
+};
+}  // namespace spec
+
 using literals::operator""_test;
 using literals::operator""_i;
 using literals::operator""_s;

test/ut/ut.cpp

@@ -1278,6 +1278,8 @@ int main() {
   {
     test_cfg = fake_cfg{};
 
+    using namespace ut::bdd;
+
     "scenario"_test = [] {
       given("I have...") = [] {
         when("I run...") = [] {
@@ -1304,6 +1306,20 @@ int main() {
     test_assert("1 == 1" == test_cfg.assertion_calls[1].expr);
   }
 
+  {
+    test_cfg = fake_cfg{};
+
+    using namespace ut::spec;
+
+    describe("describe") = [] { it("it") = [] { expect(1_u == 1u); }; };
+
+    test_assert(2 == std::size(test_cfg.run_calls));
+    test_assert("describe"sv == test_cfg.run_calls[0].name);
+    test_assert("it"sv == test_cfg.run_calls[1].type);
+    test_assert(test_cfg.assertion_calls[0].result);
+    test_assert("1 == 1" == test_cfg.assertion_calls[0].expr);
+  }
+
   {
     test_cfg = fake_cfg{};
 
@@ -1344,6 +1360,8 @@ int main() {
 
 #if (__has_builtin(__builtin_FILE) and __has_builtin(__builtin_LINE))
   {
+    using namespace ut::bdd;
+
     test_cfg = fake_cfg{};
 
     constexpr auto file = std::string_view{__FILE__};