Update every test to import `lldb_shared`.

This is necessary in order to allow third party modules to be
located under lldb/third_party rather than under the test
folder directly.

Since we're already touching every test file anyway, we also
go ahead and delete the unittest2 import and main block wherever
possible.  The ability to run a test as a standalone file has
already been broken for some time, and if we decide we want this
back, we should use unittest instead of unittest2.

A few places could not have the import of unittest2 removed,because
they depend on the unittest2.expectedFailure or skip decorators.
Removing all those was orthogonal in spirit to the purpose of this
CL, so the import of unittest2 remains in those files that were
using it for its test decorators.  Those can be addressed
separately.

llvm-svn: 251055