message: `libtock_unittest::exit_test`: give a better error message if `current_exe()` fails.
diff: @@ -157,7 +157,8 @@ impl std::str::FromStr for ExitMessage {
  // subprocess, only executing the test specified by test_name.
  #[track_caller]
  fn spawn_test(test_name: &str) -> ExitCall {
  - let output = std::process::Command::new(std::env::current_exe().unwrap())
  + let current_exe = std::env::current_exe().expect("Unable to find test executable");
  + let output = std::process::Command::new(current_exe)
  .args(std::env::args_os())
  .arg("--nocapture")
  .arg("--exact")
  