[dotest] Add @skipIfCursesSupportMissing and annotate the new gui test

Summary:
The gui command requires curses support, which can be disabled at
compile time. This patch adds the ability to detect this situation in
the test suite and skip the test accordingly.

Reviewers: teemperor, jankratochvil

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D67073

llvm-svn: 370658