[Dexter] Add DexFinishTest command to conditionally early-exit a test program

This patch adds a command, DexFinishTest, that allows a Dexter test to
be conditionally finished at a given breakpoint. This command has the
same set of arguments as DexLimitSteps, except that it does not allow a
line range (from_line, to_line), only a single line (on_line).

Reviewed By: Orlando

Differential Revision: https://reviews.llvm.org/D111988