Fix lookup path for lldb-mi

Summary:
The test suite calls realpath on the lldb executable then append "-mi" to it to find the path of the lldb-mi executable. This does not work when using CMake builds on *nix platforms. On *nix platforms when a version number is set on executables CMake generates the binary as ${name}-${version} with a symlink named ${name} pointing to it.

This results in the lldb executable being named lldb-4.0.0, and since lldb-4.0.0-mi doesn't ever match the lldb-mi executable these tests are always disabled.

This patch looks for lldb-mi in the same directory as lldb.

Reviewers: zturner, tfiala

Subscribers: ki.stfu, enlight, lldb-commits

Differential Revision: https://reviews.llvm.org/D25486

llvm-svn: 284041