tests: remove race in test-pull-pull-corruption.t

The output from the background process was not always interleaved "correctly"
with output from the foreground process.

tests/test-pull-pull-corruption.t

@@ -1,69 +1,70 b''
 Corrupt an hg repo with two pulls.
 create one repo with a long history
 
   $ hg init source1
   $ cd source1
   $ touch foo
   $ hg add foo
   $ for i in 1 2 3 4 5 6 7 8 9 10; do
   >     echo $i >> foo
   >     hg ci -m $i
   > done
   $ cd ..
 
 create one repo with a shorter history
 
   $ hg clone -r 0 source1 source2
   adding changesets
   adding manifests
   adding file changes
   added 1 changesets with 1 changes to 1 files
   updating to branch default
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ cd source2
   $ echo a >> foo
   $ hg ci -m a
   $ cd ..
 
 create a third repo to pull both other repos into it
 
   $ hg init corrupted
   $ cd corrupted
 
 use a hook to make the second pull start while the first one is still running
 
   $ echo '[hooks]' >> .hg/hgrc
   $ echo 'prechangegroup = sleep 5' >> .hg/hgrc
 
 start a pull...
 
-  $ hg pull ../source1 &
+  $ hg pull ../source1 > pull.out 2>&1 &
 
 ... and start another pull before the first one has finished
 
   $ sleep 1
-  pulling from ../source1
-  requesting all changes
   $ hg pull ../source2 2>/dev/null
   pulling from ../source2
-  adding changesets
-  adding manifests
-  adding file changes
-  added 10 changesets with 10 changes to 1 files
-  (run 'hg update' to get a working copy)
   searching for changes
   adding changesets
   adding manifests
   adding file changes
   added 1 changesets with 1 changes to 1 files (+1 heads)
   (run 'hg heads' to see heads, 'hg merge' to merge)
+  $ cat pull.out
+  pulling from ../source1
+  requesting all changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 10 changesets with 10 changes to 1 files
+  (run 'hg update' to get a working copy)
 
 see the result
 
   $ wait
   $ hg verify
   checking changesets
   checking manifests
   crosschecking files in changesets and manifests
   checking files
   1 files, 11 changesets, 11 total revisions