[Dexter] Allow tests to specify command line options

This patch adds a "DexCommandLine" command, allowing dexter tests to
specify what command line options the test should be started with. I've
also plumbed it through into the debuggers.

This eases the matter of pointing Dexter at larger tests, or controlling
different paths through a single binary from a Dexter test.

Differential Revision: https://reviews.llvm.org/D115330