[llvm] [test] Remove non-portable EISDIR test from macho-disassemble-g-dsym.test

Remove the test checking error message for 'is a directory'.  It does
not seem to serve any real purpose, and it relies on matching platform
error strings which are unpredictable and makes the test fragile.
Furthermore, it fails on NetBSD where read() works on directories,
and therefore does not return EISDIR at all.

Fixes r362141.

Differential Revision: https://reviews.llvm.org/D62773

llvm-svn: 362404