Future proof rippled by changing variable name 'requires' to 'require'

'requires' is a keyword in C++20.

https://en.cppreference.com/w/cpp/keyword/requires

src/test/jtx/require.h

@@ -74,7 +74,7 @@ public:
     void
     operator()(Env&, JTx& jt) const
     {
-        jt.requires.emplace_back(cond_);
+        jt.require.emplace_back(cond_);
     }
 };