[LLD][ELF] Correct test temporary file paths

In 8031785f the temporary object being built was moved to %t/main.o,
but not all run lines were updated to reflect this. Observe the failure
on this buildbot:

  http://lab.llvm.org:8011/#/builders/5/builds/3646/steps/9/logs/stdio

It might pass locally for some people due to a stale %t.o hanging around
the build directory.