Add -wait-for-browser for Linux and macOS as well.

[juj]

In previous PR #25644 I added -wait-for-browser use on Windows to help single-threaded Windows runner not leave behind stale browser windows.

For some reason, it was not passing on CircleCI on Linux. Testing this again to see what the failure is.

In local testing with a test script:

import subprocess, sys, time

cmd = ['C:\\Program Files\\Mozilla Firefox\\firefox.exe', '-new-instance', '-profile', sys.argv[1], '-wait-for-browser', 'https://wiki.mozilla.org/Firefox/CommandLineOptions']
print('Launching browser: ' + ' '.join(cmd))
proc = subprocess.Popen(cmd)

for i in range(10):
  if proc.poll() is not None:
    print('Oops: process already quit, so we did not get a process handle to keep track of the browser instance.')
    sys.exit(0)
  time.sleep(1)

if proc.poll() is None:
  print('terminating browser process')
  proc.terminate()
time.sleep(2)
if proc.poll() is None:
  print('terminating did not work. killing browser process')
  proc.kill()

print('All done')

I am unable to find a behavioral difference between Windows and Linux.

So testing this out again on CircleCI.

[juj]

Ok, figured out the issue. Reported as https://bugzil.la/1996614