Fix GDBRemoteCommunicationClientTest.TestPacketSpeedJSON

The mock server was listening for only one packet (I forgot to put a loop around
it), which caused the client to stall in debug builds, as the timeout there is
1000 seconds. In case of a release builds the test would just silently succeed as
the tested function does not check or report errors (which should be fixed).

This fixes the test by adding the server loop. Since the test was taking quite a
long time now (8s), I have added a parameter to control the amount of data sent
(default 4MB), and call it with a smaller value in the test, to make the test run
faster.

llvm-svn: 285992