[mach-o]: remove dylib install-name test with a.out

Unfortunately, the creation of (the default) output file, a.out races with all
the other tests in this directory. When the wrong one is read by macho-dump,
the test fails.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@212269 91177308-0d34-0410-b5e6-96231b3b80d8