Skip tests that are intermittent on Linux, fix gdb-remote port-grabbing code.

Marked skipped for Linux:
TestCallStopAndContinue
TestConvenienceVariables
TestStopHookMultipleThreads

Fixed up gdb-remote port-grabbing code to use a random port in a wide range,
and to allow that to fail more gracefully.  This appears to have solved some
gdb-remote intermittent failing behavior.

llvm-svn: 212662