testsuite: add dmesg-grep.py helper script

Problem: Sometimes it is useful during testing to wait for a given
pattern to appear in the Flux dmesg logs, but waiting for a matching
line to appear in `flux-dmesg(1)` output using the shell is brittle
and inconvenient.

Copy the dmesg-grep.py script from flux-core for this purpose.

t/scripts/dmesg-grep.py

@@ -0,0 +1,69 @@
+#!/usr/bin/env python3
+###############################################################
+# Copyright 2019 Lawrence Livermore National Security, LLC
+# (c.f. AUTHORS, NOTICE.LLNS, COPYING)
+#
+# This file is part of the Flux resource manager framework.
+# For details, see https://github.com/flux-framework.
+#
+# SPDX-License-Identifier: LGPL-3.0
+###############################################################
+#
+#  Follow Flux dmesg output until a line matches a pattern
+#
+import argparse
+import re
+import sys
+
+import flux
+from flux.constants import FLUX_RPC_STREAMING
+from flux.core.watchers import TimerWatcher
+
+parser = argparse.ArgumentParser(
+    description="watch the flux dmesg log for a given pattern"
+)
+parser.add_argument(
+    "-t",
+    "--timeout",
+    help="Timeout with error after some number of seconds",
+    metavar="SEC",
+    type=float,
+    default=1.0,
+)
+parser.add_argument(
+    "-v",
+    "--verbose",
+    help="Emit each line of dmesg output, not just first matching",
+    action="count",
+    default=0,
+)
+parser.add_argument("pattern")
+args = parser.parse_args()
+
+
+def timer_cb(h, watcher, revents, _arg):
+    print("Timeout!", file=sys.stderr)
+    h.reactor_stop_error()
+
+
+def dmesg_cb(rpc, pattern, verbose=False):
+    buf = rpc.get_str()
+    match = pattern.search(buf)
+    if match:
+        print(buf)
+        rpc.flux_handle.reactor_stop()
+    elif verbose:
+        print(buf)
+    rpc.reset()
+
+
+pattern = re.compile(args.pattern)
+
+h = flux.Flux()
+
+rpc = h.rpc("log.dmesg", {"follow": True}, flags=FLUX_RPC_STREAMING)
+rpc.then(dmesg_cb, pattern, verbose=args.verbose)
+
+TimerWatcher(h, args.timeout, timer_cb).start()
+if h.reactor_run() < 0:
+    sys.exit(1)