Fix tests on cmake-based OS X after rL248338

See:
https://llvm.org/bugs/show_bug.cgi?id=24926

for details.  On OS X, when LLDB.framework is
not part of the lldb.dylib path, the supporting
executable path is resolved to be the bin directory
sitting next to the lib directory with the dylib
lives.  Not a perfect solution, but we also can't
base it on the executable path since both Python
and the lldb driver can be the executable.

llvm-svn: 248545