Add microsecond accuracy to log timestamps

This is useful while debugging and trying to check events across other
outputs such as pcap files, process logs, etc.

Change-Id: I43bb5c6e9977189251802bc2b078c52eb046bab8
diff --git a/src/osmo_gsm_tester/process.py b/src/osmo_gsm_tester/process.py
index d24bf3d..4b68ef5 100644
--- a/src/osmo_gsm_tester/process.py
+++ b/src/osmo_gsm_tester/process.py
@@ -21,6 +21,7 @@
 import time
 import subprocess
 import signal
+from datetime import datetime
 
 from . import log, test, event_loop
 from .util import Dir
@@ -55,7 +56,7 @@
         path = self.run_dir.new_child(name)
         f = open(path, 'w')
         self.dbg(path)
-        f.write('(launched: %s)\n' % time.strftime(log.LONG_DATEFMT))
+        f.write('(launched: %s)\n' % datetime.now().strftime(log.LONG_DATEFMT))
         f.flush()
         self.outputs[name] = (path, f)
         return f