llvm-isel-fuzzer: Work around BUILD_SHARED_LIBS testing issues

Building with BUILD_SHARED_LIBS makes it tricky to copy around
executables at will, since they won't be able to find the LLVM
libraries any more. This makes testing a feature that's based on the
executable name problematic, so we'll just disable these two tests in
that configuration.

We could potentially fix this by symlinking the lib directory into the
test directory, but that wouldn't work on windows, and losing testing
on windows would be far worse than losing testing on a configuration
that's barely even supported.

llvm-svn: 315599