test-commandserver: change way of triggering early crash

Future patches will move the logging facility out of the server class, so
cmdserver.log can't be (ab)used for this purpose. Instead, let's hook the
factory function to raise exception.

tests/test-commandserver.t

@@ -846,9 +846,18 b' unix domain socket:'
  if server crashed before hello, traceback will be sent to 'e' channel as
  last ditch:
 
+  $ cat <<'EOF' > ../earlycrasher.py
+  > from mercurial import commandserver, extensions
+  > def _serverequest(orig, ui, repo, conn, createcmdserver):
+  >     def createcmdserver(*args, **kwargs):
+  >         raise Exception('crash')
+  >     return orig(ui, repo, conn, createcmdserver)
+  > def extsetup(ui):
+  >     extensions.wrapfunction(commandserver, b'_serverequest', _serverequest)
+  > EOF
   $ cat <<EOF >> .hg/hgrc
-  > [cmdserver]
-  > log = inexistent/path.log
+  > [extensions]
+  > earlycrasher = ../earlycrasher.py
   > EOF
   >>> from hgclient import bprint, check, readchannel, unixserver
   >>> server = unixserver(b'.hg/server.sock', b'.hg/server.log')
@@ -863,13 +872,13 b' unix domain socket:'
   ...             break
   >>> check(earlycrash, server.connect)
   e, 'Traceback (most recent call last):\n'
-  e, "(IOError|FileNotFoundError): .*" (re)
+  e, 'Exception: crash\n'
   >>> server.shutdown()
 
   $ cat .hg/server.log | grep -v '^  '
   listening at .hg/server.sock
   Traceback (most recent call last):
-  (IOError|FileNotFoundError): .* (re)
+  Exception: crash
   killed!
 #endif
 #if no-unix-socket