TestMultilineExpr: validate evaluation for expressions that span multiple lines

Summary:
When LLDB successfully parses a command (like "expression" in this case) and determines incomplete input, the user can continue typing on multiple lines (in this case "2+3"). This should provide the correct result.
Note that LLDB reverts input from the additional lines, so they are not present in the output.

Reviewers: vsk, davide, aprantl

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D52270

llvm-svn: 343860