tests: gdbstub: Add GDB remote target parameter

Add `gdb_target_remote` test parameter for GDB `target remote`
command instead of its hardcoded value to allow different types
of gdbstub serial interfaces as well as different TCP ports in
gdbstub test suites possibly run in parallel on the same host.

Move all GDB log configuration parameters from GDB script to
the fixture code.

Signed-off-by: Dmitrii Golovanov <dmitrii.golovanov@intel.com>