I'll take this one up to go with Core38.
I dont think start/stop bluetoothd will bring down the physical interface (and along with it, the BT radio). we need the BT equivalent of "ifconfig wlan0 down"

https://chromium-review.googlesource.com/c/chromiumos/overlays/chromiumos-overlay/+/1187837
https://chromium-review.googlesource.com/c/chromiumos/third_party/autotest/+/1187564

I've only tested on eve and samus, will need help to test this on all boards 
(lock wificells for boards covering Marvell, QCA etc) and run this test locally. 
Harpreet, can your team help?

kirtika@ - Instead of running this autotest on ALL boards, I suggest we spot check on few boards with few different kernel & wifi chipset combinations. We can help with this.

We can either do this on M69 or M70 using recent builds with the fix. Any preference?

Sorry, that was my bad to say "all boards". I did mean enough to cover most SoC/Wifi chip combinations. I did find some flakiness on scarlet. 

Both R69 and R70 are fine. Be sure that the OS version you pick up for testing contains the fix CL. You can find all fix CLs here:
https://chromium-review.googlesource.com/q/hashtag:%22bt-release-antenna%22+(status:open%20OR%20status:merged)

Spot checked on boards with a different kernel and wifi chipset. Result details in the following spreadsheet.  https://docs.google.com/spreadsheets/d/1q0tXPkSpgJ2grfsPL-BO2mqpNRV6FjtcmfFDxDyF8AM/edit?usp=sharing 
Everything looks good. 

side note: If you have completed testing and have any devices locked in autotest,  Please unlock it.  

Thanks Aashutosh!
I've unlocked the scarlet I was using to test this. 
Now waiting on code review to submit this. 

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/9ddc7fc4b05e1a9a7e11bfd203f76ee6df4c745d

commit 9ddc7fc4b05e1a9a7e11bfd203f76ee6df4c745d
Author: Kirtika Ruchandani <kirtika@google.com>
Date: Sun Dec 02 05:19:14 2018

autotest-tests-shill: Add network_WiFi_BT_AntennaCoex

Add an entry for an antenna co-existence test.

BUG=b:79233533, chromium:876920
TEST=Ran this test on samus, eve, nyan_big and scarlet.

Change-Id: I629d6c74486992fc4db22a1510a56bd6b4b751e4
Signed-off-by: Kirtika Ruchandani <kirtika@google.com>
Reviewed-on: https://chromium-review.googlesource.com/1187837
Commit-Ready: Kirtika Ruchandani <kirtika@chromium.org>
Tested-by: Kirtika Ruchandani <kirtika@chromium.org>
Reviewed-by: Neeraj Poojary <npoojary@google.com>
Reviewed-by: Brian Norris <briannorris@chromium.org>

[modify] https://crrev.com/9ddc7fc4b05e1a9a7e11bfd203f76ee6df4c745d/chromeos-base/autotest-server-tests-shill/autotest-server-tests-shill-9999.ebuild

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/third_party/autotest/+/c46e2e0f813332b64ecf5b6f2ce5f4a7f6f0c418

commit c46e2e0f813332b64ecf5b6f2ce5f4a7f6f0c418
Author: Kirtika Ruchandani <kirtika@google.com>
Date: Sun Dec 02 05:19:14 2018

autotest: wifi: Add network_WiFi_BT_AntennaCoex

b:79233533 generated a large number of "wifi doesn't work until reboot"
feedback reports. The root cause was wifi not being able to
initialize/calibrate because BT was holding the shared antenna hostage.
Prevent this with a simple test that tests various sequences of wifi
off/on mixed with bt off/on. Note that there is no graceful failure with
this test - if the wifi interface is lost, shill decides to die with
LOG(FATAL), following which dhcpcd also kills itself (because no shill..) and
that breaks our ethernet control connection to the DUT. The fix for that
situation should be tackled in chromium:698457.

CQ-DEPEND=CL:1187837
BUG=b:79233533, chromium:876920
TEST=Ran this test on eve, samus, nocturne and bobba.
Removed the fix (CL:1162876) on 4.4 kernel and tested that this test borks
the eve DUT without the CL added.

Change-Id: I127de09881462c236ad07635fc1a8b841ecdc8cb
Reviewed-on: https://chromium-review.googlesource.com/1187564
Commit-Ready: Kirtika Ruchandani <kirtika@chromium.org>
Tested-by: Kirtika Ruchandani <kirtika@chromium.org>
Reviewed-by: Brian Norris <briannorris@chromium.org>

[add] https://crrev.com/c46e2e0f813332b64ecf5b6f2ce5f4a7f6f0c418/server/site_tests/network_WiFi_BT_AntennaCoex/network_WiFi_BT_AntennaCoex.py
[add] https://crrev.com/c46e2e0f813332b64ecf5b6f2ce5f4a7f6f0c418/server/site_tests/network_WiFi_BT_AntennaCoex/control