[libc++] Simplify selftest to avoid passing arguments to it

[ldionne]

Summary

This makes the test pass on executors that don't support passing additional
arguments.

libcxx/test/libcxx/selftest/sh.cpp/substitutions.sh.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // Make sure we have access to the following substitutions at all times:
 // - %{cxx}
 // - %{flags}
 // - %{compile_flags}
 // - %{link_flags}
 // - %{exec}
 
 // RUN: %{cxx} %s %{flags} %{compile_flags} %{link_flags} -o %t.exe
-// RUN: %{exec} %t.exe "HELLO"
+// RUN: %{exec} %t.exe
 
 #include <cassert>
-#include <string>
 
-int main(int argc, char** argv) {
-  assert(argc == 2);
-
-  std::string arg = argv[1];
-  assert(arg == "HELLO");
+int main(int argc, char**) {
+  assert(argc == 1);
   return 0;
 }