Add "not" to an llvm-symbolizer test that expects to fail

In a7b209a6, llvm-symbolizer was adjusted to return a failure status
code when it produced an error, to flag up DWARF parsing problems. The
test for missing PDB file is analogous, and returns a failure status now
too.

This should fix the llvm-clang-win-x-armv7l buildbot croaking:

  http://lab.llvm.org:8011/#/builders/60/builds/77