killdaemons: close pid file before killing processes

With #serve enabled on Windows, I was getting occasional stacktraces like this:

  Errored test-hgweb-json.t: Traceback (most recent call last):
    File "./run-tests.py", line 724, in run
      self.tearDown()
    File "./run-tests.py", line 805, in tearDown
      killdaemons(entry)
    File "./run-tests.py", line 540, in killdaemons
      logfn=vlog)
    File "...\tests\killdaemons.py", line 94, in killdaemons
      os.unlink(pidfile)
  WindowsError: [Error 32] The process cannot access the file because it is
     being used by another process: '...\\hgtests.zmpqj3\\child80\\daemon.pids'

Adrian suggested using util.posixfile, which works.  However, the 'mercurial'
package isn't in sys.path when invoking run-tests.py, and it isn't clear that
hacking[1] it in is a good thing (especially for test-run-tests.t, which uses an
installation in a temp folder).

I tried using ProcessMonitor to figure out what the other process is, but that
monitoring slows things down to such a degree that the issue doesn't occur.  I
was ready to blame the virus scanner, but it happens without that too.

Looking at the code, I don't see anything that would have the pid file open.
But I was able to get through about 20 full test runs without an issue with this
minor change, whereas before it was pretty certain to hit this at least once in
two or three runs.

[1] https://www.mercurial-scm.org/pipermail/mercurial-devel/2017-May/097907.html

tests/killdaemons.py

@@ -78,7 +78,8 b' def killdaemons(pidfile, tryhard=True, r'
         logfn = lambda s: s
     # Kill off any leftover daemon processes
     try:
-        fp = open(pidfile)
+        pids = []
+        with open(pidfile) as fp:
         for line in fp:
             try:
                 pid = int(line)
@@ -88,8 +89,9 b' def killdaemons(pidfile, tryhard=True, r'
                 logfn('# Not killing daemon process %s - invalid pid'
                       % line.rstrip())
                 continue
+                pids.append(pid)
+        for pid in pids:
             kill(pid, logfn, tryhard)
-        fp.close()
         if remove:
             os.unlink(pidfile)
     except IOError: