[cxx-interop] Add a MutableCollection test that relies on std::string being Comparable

This is possible after swiftlang#76223.

Author: egorzhdan

test/Interop/Cxx/stdlib/use-std-vector.swift

@@ -97,6 +97,11 @@ StdVectorTestSuite.test("VectorOfString as MutableCollection") {
     expectEqual(v[0], std.string("xyz"))
     expectEqual(v[1], std.string("abc"))
     expectEqual(v[2], std.string("ijk"))
+
+    v.sort() // Swift function
+    expectEqual(v[0], std.string("abc"))
+    expectEqual(v[1], std.string("ijk"))
+    expectEqual(v[2], std.string("xyz"))
 }
 #endif