SITL --add-param-file fails with multiple vehicles

[eschutz]

Bug report

Issue details

Passing a param file to sim_vehicle.py with --count > 1 causes ArduPlane SITL to fail to start.

Reproduce by running the following in the same directory as a valid parameter file my_plane.parm:
sim_vehicle.py -v plane --add-param-file=my_plane.parm --count 2

Running ArduPlane on its own (executed verbatim from the sim_vehicle.py logs, see below), and connecting with MAVProxy from another terminal, I get:
<ardupilot directory>/build/sitl/bin/arduplane -S --model plane --speedup 1 --slave 0 --defaults ../../../ardupilot/Tools/autotest/models/plane.parm,my_plane.parm --sim-address=127.0.0.1 -I0

Setting SIM_SPEEDUP=1.000000
Starting sketch 'ArduPlane'
Starting SITL input
Using Irlock at port : 9005
bind port 5760 for SERIAL0
SERIAL0 on TCP port 5760
Waiting for connection ....
Connection on serial port 5760
PANIC: Failed to load defaults from ../../../ardupilot/Tools/autotest/models/plane.parm,my_plane.parm

Running: sh dumpstack.sh 2879426 >dumpstack.sh_arduplane.2879426.out 2>&1
Failed
Running: sh dumpcore.sh 2879426 >dumpcore.sh_arduplane.2879426.out 2>&1
Failed

The PANIC line seems to indicate there's a problem with the parameter file path. When running the same command with one level of .. removed: <ardupilot directory>/build/sitl/bin/arduplane -S --model plane --speedup 1 --slave 0 --defaults ../../ardupilot/Tools/autotest/models/plane.parm,my_plane.parm --sim-address=127.0.0.1 -I0, ArduPlane runs as expected.
This suggests the problem lies at this line of code

ardupilot/Tools/autotest/sim_vehicle.py

Line 798 in 4d31a73

path = os.path.join("..", path)

Note that this is in the same region that seems to cause another bug with multiple agents #28826.

Version
4df758f

Logs
Output of sim_vehicle.py:

SIM_VEHICLE: Using defaults from (../../../ardupilot/Tools/autotest/models/plane.parm)
SIM_VEHICLE: Adding parameters from (my_plane.parm)
SIM_VEHICLE: Run ArduPlane
SIM_VEHICLE: "<ardupilot directory>/Tools/autotest/run_in_terminal_window.sh" "ArduPlane" "<ardupilot directory>/build/sitl/bin/arduplane" "-S" "--model" "plane" "--speedup" "1" "--slave" "0" "--defaults" "../../../ardupilot/Tools/autotest/models/plane.parm,my_plane.parm" "--sim-address=127.0.0.1" "-I0"
SIM_VEHICLE: Run ArduPlane
SIM_VEHICLE: "<ardupilot directory>/Tools/autotest/run_in_terminal_window.sh" "ArduPlane" "<ardupilot directory>/build/sitl/bin/arduplane" "-S" "--model" "plane" "--speedup" "1" "--slave" "0" "--defaults" "../../../ardupilot/Tools/autotest/models/plane.parm,my_plane.parm" "--sim-address=127.0.0.1" "-I1"
RiTW: Starting ArduPlane : <ardupilot directory>/build/sitl/bin/arduplane -S --model plane --speedup 1 --slave 0 --defaults ../../../ardupilot/Tools/autotest/models/plane.parm,my_plane.parm --sim-address=127.0.0.1 -I0
RiTW: Starting ArduPlane : <ardupilot directory>/build/sitl/bin/arduplane -S --model plane --speedup 1 --slave 0 --defaults ../../../ardupilot/Tools/autotest/models/plane.parm,my_plane.parm --sim-address=127.0.0.1 -I1
SIM_VEHICLE: Run MavProxy
SIM_VEHICLE: "mavproxy.py" "--out" "127.0.0.1:14550" "--master" "tcp:127.0.0.1:5760" "--sitl" "127.0.0.1:5501" "--out" "127.0.0.1:14560" "--master" "tcp:127.0.0.1:5770" "--sitl" "127.0.0.1:5511"
Connect tcp:127.0.0.1:5760 source_system=255
[Errno 111] Connection refused sleeping
Connect tcp:127.0.0.1:5770 source_system=255

<mavinit logs>

Log Directory: 
Telemetry log: mav.tlog
Waiting for heartbeat from tcp:127.0.0.1:5760
MAV> link 1 down
link 2 down