Accumulate gdbserver output before checking for target port

[jonahgraham]

Because the needed output from gdbserver may not arrive as a single 'data' event on stdout we need to accumulate the output before passing it to the regex checker.

Fixes #299

[jonahgraham]

I had a failure with the earlier version of this change, so I added this extra output event to understand what is happening and now the test passes - still a bit of experimentation to make sure that this accumulation is implemented correctly.

[jonahgraham]

Found the issue - the default regex is Listening on port ([0-9]+) which means when the input (e.g. Listening on port 12345) comes in a character at a time, the regex matches Listening on port 1 and thinks the port is 1.

[jonahgraham]

This change updates the regex from Listening on port ([0-9]+) to Listening on port ([0-9]+)\r?\n, all the docs for default value of serverPortRegExp say "defaults to matching a string like 'Listening on port 41551'":

cdt-gdb-adapter/src/GDBTargetDebugSession.ts

Line 67 in 3e91a67

// defaults to matching a string like 'Listening on port 41551' which is what gdbserver provides

which means I don't think it should be a problem to change the default implementation here.