message: do not validate start-fn code
diff: @@ -103,6 +103,8 @@ fn miri_pass(sysroot: &Path, path: &str, target: &str, host: &str, need_fullmir:
  flags.push("-Dwarnings -Dunused".to_owned()); // overwrite the -Aunused in compiletest-rs
  if have_fullmir() {
  flags.push("-Zmiri-start-fn".to_owned());
  + // start-fn uses ptr::read, and so fails validation
  + flags.push("-Zmiri-disable-validation".to_owned());
  }
  if opt {
  flags.push("-Zmir-opt-level=3".to_owned());
  