Refactored gdb remote protocol testing sequence handling.

Pushed gdbremote protocol sequence expectations into a separate
class and defer matching of llgs/debugserver output to the
sequence entry.  Pre-step to adding regex matching and
grouped content collecting.

No longer require anything before the read/send packet
portion of slurped-up log lines used for setting up
gdb remote test sequences.  Several packet logging options
produce a wide range of content before the read/send packet.

Added helpers to TestLldbGdbServer to clean up test setup
and test expectations matching.

llvm-svn: 207998