tests: teach `killdaemons` on Windows to use an exit code provided by a caller

Right now, there are several tests that use `signal.SIGKILL`, which isn't a
thing on Windows.  The `killdaemons` script approximates this by forcibly
terminating the process.  There's a minor difference in that `signal.SIGKILL`
results in the test capturing an exit code of 137, and the `killdaemons` victim
doesn't record a code (as though it exited with 0).  Since the exit code line
couldn't be conditionalized the last time I checked, let's just allow the caller
to simulate the same exit code, and avoid conditionalizing the tests.

tests/killdaemons.py

@@ -56,7 +56,12 if os.name == 'nt':
             if r == WAIT_OBJECT_0:
                 pass  # terminated, but process handle still available
             elif r == WAIT_TIMEOUT:
-                _check(ctypes.windll.kernel32.TerminateProcess(handle, -1))
+                # Allow the caller to optionally specify the exit code, to
+                # simulate killing with a signal.
+                exit_code = int(os.environ.get("DAEMON_EXITCODE", -1))
+                _check(
+                    ctypes.windll.kernel32.TerminateProcess(handle, exit_code)
+                )
             elif r == WAIT_FAILED:
                 _check(0)  # err stored in GetLastError()