[libc++] Split off debug tests that were missed by ce1365f8f7 into test/libcxx

Also, some tests had multiple death tests in them, so split them into
separate tests instead. The second death test would obviously never
get run, because the first one would kill the program before.

libcxx/test/std/strings/basic.string/string.access/back.pass.cpp

@@ -11,10 +11,6 @@
 // const charT& back() const;
 //       charT& back();
 
-#ifdef _LIBCPP_DEBUG
-#define _LIBCPP_ASSERT(x, m) ((x) ? (void)0 : std::exit(0))
-#endif
-
 #include <string>
 #include <cassert>
 
@@ -50,13 +46,6 @@ int main(int, char**)
     test(S("1234567890123456789012345678901234567890"));
     }
 #endif
-#ifdef _LIBCPP_DEBUG
-    {
-        std::string s;
-        (void) s.back();
-        assert(false);
-    }
-#endif
 
-  return 0;
+    return 0;
 }