test-logtoprocess: use cat to wait for outputs

Commands started by logtoprocess are running asynchronously. To be able to
test the output, we need to block and wait for the output.

The patch uses "| cat" to wait for such "asynchronous" outputs, to make the
test more reliable. I have also written a short notice at the top, hopefully
people would be aware of the pitfall when changing the test.

tests/test-logtoprocess.t

@@ -1,3 +1,7 b''
+ATTENTION: logtoprocess runs commands asynchronously. Be sure to append "| cat"
+to hg commands, to wait for the output, if you want to test its output.
+Otherwise the test will be flaky.
+
 Test if logtoprocess correctly captures command-related log calls.
 
   $ hg init
@@ -34,9 +38,8 b' Test if logtoprocess correctly captures '
 Running a command triggers both a ui.log('command') and a
 ui.log('commandfinish') call. The foo command also uses ui.log.
 
-Use head to ensure we wait for all lines to be produced, and sort to avoid
-ordering issues between the various processes we spawn:
-  $ hg foo | head -n 17 | sort
+Use sort to avoid ordering issues between the various processes we spawn:
+  $ hg foo | cat | sort
   
   
   
@@ -66,5 +69,5 b' Confirm that logging blocked time catche'
   > logblockedtimes=True
   > EOF
 
-  $ hg log
+  $ hg log | cat
   uiblocked stdio [0-9]+.[0-9]* ms command [0-9]+.[0-9]* ms (re)